I once had a friend with a carefully laid-out network of associates. Friends, classmates, coworkers, everyone she knew, she kept up with. She made reminders for herself, every month or so, to reaffirm that connection with the people she didn't see often. She'd call them up and chat for a bit, or send them an email, just to make sure they remembered her and their relationship with her. This was part a need for social contact and part survival technique; if she needed a new job, she had twenty people she could call who might potentially have leads for her. If she got to that interview, there were a dozen individuals to act as references. It was a well-constructed safety net, and I remember feeling exhausted just hearing her talk about it.
I am the product of an extrovert (my mother) and an introvert (my father). I take after my father. Too much social activity drains me, and if I am low on my Ability to Tolerate Shit to start with, just being in a crowded area can cause a complete meltdown. Parties, for most of my life, have been things I am forced to go to because my mother said so. I was that kid who brought a book to every social gathering, because it was so much easier to sit in a corner and read than be forced to make small talk with people. This introversion is not a hard and fast rule--if I know someone well and genuinely like/trust them, I can spend a lot more time with them before needing a break. But I am still largely passive when it comes to social interaction. On the one hand, I never, ever assume that someone actually wants to spend time with me. I will never invite myself to a gathering or activity of any sort, even if people I consider friends are making the plans while standing on either side of me. I have hosted a number of gatherings at my house, but even those were at the request of friends (Travis has very convincing sad puppy-dog eyes). Had said friends not kept suggesting, I would probably not have hosted any sort of party. The amount of social interaction in which I have participated in the past year or two is more than anything I've done since high school, when much of it was forced (when you're in marching band, you spend a LOT of time with the same group of people). Most of the socialization in my life is passive and internet-based. See also: this journal.
If I follow you on LJ, it means I think you are interesting. Maybe I am a fan of something you've done, or made, or are currently in the process of making--it is always rather sad to realize that someone who creates amazing things is actually boring or kind of a jerk, so when it turns out that they are awesome, I often friend them. Maybe you posted something in a community of which I was a part, and I said to myself, Self, this person has a perspective on things with which I agree. 95% of the time, it was probably something funny and/or geeky. I have a 'type', yes. Even when it comes to acquaintences.
If we are friends on Facebook, it means either I know you in meatspace, or I have known you for a while on the interwebs and am fairly convinced you are an actual human.
In all of these cases, I read what you post. I may miss something if I go through a long period without internet access, or I may attribute your post to someone else accidentally (there are a few people with similar writing styles, similar userpics, similar names, or all three, which can lead to confusion on my part), but I do read. Except for those rare days when I feel quite chatty, I rarely comment unless I have something new to contribute to the situation, or it is a case where having a new perspective isn't necessarily required (the usual congratulations/condolences sort of response).
There are a handful of folks I speak to on AIM. Some are meatspace friends, some are internet, some started out as one and became the other. I don't actually talk to people all that often; my impulsive need to blather aimlessly is curbed quite nicely by Twitter, which I rarely check for replies or followers.
This system works for me until people start vanishing. My unwillingness to put forth that extra effort tends to kill any connection. Oh, you're starting a new blog and not importing posts to LJ? Unless you're one of the most fascinating people I know and are talking about something I find really interesting, odds are I won't bother to check this new webpage often, if at all. You're leaving Facebook, and you're not someone I see every week due to school or a job or some such? We will probably never talk again. It is not a malicious thing; it would be silly to resent you for leaving a social networking site. I simply do not have the energy to try and keep up with you.
This whole process has its problems--truly close connections are few and far between (which works out all right, really, since I don't trust easily), and the rare bouts of social interaction don't necessarily intersect with times when I desperately want to be around people. We're not even going to discuss the issues regarding romantic relationships, since it's looking like those are permanently off the table anyway.
But it works. It functions, more or less.
And it's why it makes me a bit sad to see people leaving LJ, or deleting their Facebooks. I understand, and do what you gotta do.
But I will miss you.