As part of my combination job search and software project promotion I've made it my goal to revitalized my my LinkedIn network. I signed up 7 years ago but just never used it. I began with a very modest ~30 connections and within 14 days I reached 1300 1st degree connections, all with minimal effort. Let me share with you the "magic" of how I did it.
But first, a major caveat. Creating a big network requires you to accept the axiom put forward by Will Rogers that, “A stranger is just a friend I haven't met yet.” The premise of LinkedIn, and most social media sites, is that you create your network to reflect your real world relationships, and while that would be nice, the reality is that it's never going to happen. The boundaries of human relationships are fuzzy -- the line between friends, acquaintances, long lost acquaintances, and friendly strangers is blurry. And people, being naturally social, always seek to make new friends, new acquaintances for personal and professional reasons. So, if your LinkedIn policy is to limit your network to only people you truly know, then your network will likely be and remain fairly small. Dunbar's number is a theorized number suggesting humans can only maintain a real cognitive connection with 100-250 others. As most people do not have thousands and thousands of people they directly know well in real life to invite, if you want a larger network you will need to make connections with friends you haven't met yet.
Invite Your Real World Connections First
Before you begin trying to expand your network with people you may not know, first make sure to spend a few days to a week connecting with those you do know, using LinkedIn's recommendations to allow you to find all those connections which you may not initially directly have access to or remember. For example, you may remember a coworker John A. at the last company you worked for, but not remember anyone else. But once John A. accepts your invite LinkedIn will start to suggest the names of others who worked there, too. This will jog your memory and you can reconnect with those others, and keep expanding your real world connections out in this manner. Once you embark on adding virtual connections the usefulness of the recommendation system will quickly drop to zero and you'll likely never see another useful real world recommendation again! So make sure you do this first!
Do Not Invite (Just Any) Random Strangers
It is critical that you do not spam random strangers with LinkedIn requests. Your LinkedIn account will be suspended! When anyone receives a LinkedIn invite they have three options: Accept, Archive, or I Don't Know. Accept means just what you think it does. Archive means do not accept the invite, but file it away so I don't see it any more. "I Don't Know" theoretically means "I Don't Know [this Person in Real Life]", but what it really means is "I Don't [Want to] Know [this Person]". If you have too many people say "I Don't Know" then your account will be suspended. So it is very important you do not invite those who will "I Don't Know" you. Fortunately a great many friendly strangers within LinkedIn do want to know you and indicate as such in their profiles, a common acronym for these people is LION (LinkedIn Open Network), you will see it in their name line and in their profiles. But just because they say LION does not necessarily mean they will not "I Don't Know" (IDK) you. While most LIONs will NOT IDK you, some may use the LION phrase without appreciating what it implies, and others may simply not accept what it implies. So the way to stay safe is to look in their profile for a phrase like, "I never IDK." If you're going to invite a friend you haven't met yet, make sure they never IDK!
Indicate your Openness in your Profile
Now that you know about LIONs and IDKs, you can put those elements in your summary and you will see an increase in the number of incoming invitations to connect you will get. You are saying you are open for others to invite you, and they will. But this alone will not get you many connections fast. Your profile is just too invisible. At best you might get a slow trickle of 0-2 invites a day just by including these things in your summary/profile.
You may wish to remove these indicators from your profile later, once you feel you have enough connections. I can only assume that many people make judgments based on whether a profile seems more populated by real world connections or virtual ones, and putting LION or IDK in your profile may may lower your profile's perceived value to those people.
Boost Your Visibility by Joining Groups
LinkedIn allows people to see the detailed profiles of others when they are 1st degree connections, 2nd degree connections, or share group membership. This last item is very important. If you join a group of 15,000 you have effectively grown your network by that many people, you become more visible to them and they to you. I recommend joining as many groups as reasonable to your interests, favoring the larger groups (each group indicates how many members are in it and how active it is, there is little value in joining a group of 150 people who have no active conversations).
Join LION (and related) Groups
If your goal is to make friends fast one key is to join LION-related groups. Almost all of them have posts which are purely there for the purpose of making friends. The way it works is someone creates a post and says, "Let's network, post your email as a comment!" and people do just that. The comment threads are often 2,000+ comments long, which means (roughly, there are repeats) 2,000 people are indicating an interest to befriend anyone, and sharing their email address so that you can invite them.
I won't detail the process, but generally speaking here is how you can use these posts/comments if you so choose.
Step 1: Expand entire comment section.
Keep expanding all the comment pages in the given post. As you scroll down it will keep expanding the content, until every so often it requires that you click "SHOW MORE COMMENTS". Keep repeating until you run out or get tired.
Step 2: Copy or save the HTML of that post and the shown comments.
Step 3: Extract the email addresses.
With the content of the page you can now extract the emails into a nice clean list. You can use online tools to do this (such as this one), your favorite text editor (using regular expression), or other tools.
Step 4: Import your email address list into LinkedIn Contact List.
The next step will be to send a message to all these people saying that you too would like to connect. To do this you need to import their email addresses into LinkedIn's contact list. If you have any contacts already in that list you should delete them, otherwise how will you know who is who? Deleting is a pain because the contact list only lets you delete in small batches (under ~100?) otherwise you get an error.
Once you have cleared your contact list, it's time to import the new one. For that click the add contact icon in the top right and then on the page that follows choose the upload contact list option, seen highlighted in the screen capture. Then just give it your big list of emails.
Step 5: Contact your Contacts!
Now that the contacts are in the list you need to select them, choose a custom message, and invite them. I strongly recommend the message you choose reference how you found them and why you are connecting. Something like, "I found your email listed on the [Insert Name of Original Post] thread as someone interested in more connections, I would like to add you as mine. Thanks." And you'll need to repeat this quite a few times because they don't let you email to all at once. I did it by letter, first all the As, then the Bs, etc.
And voila, the connections will begin to come pouring in. But keep in mind, having connections doesn't alone do very much for you! You'll need to begin posting, interacting, etc. to leverage the network you build.
NOTE: LinkedIn imposes a fixed limit on outgoing invites (I believe it is 3,000), so once you hit that limit you won't be able to invite any more people. Others can still invite you, though. You can contact customer service to try and get your limit raised but they may not accommodate you. From what I have read they look at your activity and ratios for IDKing, etc. And withdrawing invites from your InMail Sent folder does not restore invite credits.
Update: A week later and I'm now at 2,000 connections. There has been a steady stream of acceptances since my initial invite distribution, and a subsequent smaller one of about 600 seemed to keep the connections coming. It remains to be seen just how useful this network will prove to be, to what use I can put it in the promotion of myself, my projects, etc. I suspect it will have severely limited potential in those respects.