Have you ever run into a childhood friend?
It's kind of awkward.
First, there's the recognition.
You say hello, exchange casual greetings, and perhaps bring up a fond memory or two.
Then there's the obligatory "So, what are you up to now?" question, and you don't really know how to answer, so you make some generic small talk.
Within a couple of minutes, you get to the awkward silence.
You clearly don't have anything to say to each other, but you kind of feel obligated to say something, and you've already asked all the generic questions, but you can't really get more specific because you obviously don't know one another anymore...
And that's why there's Facebook.
You can keep up with your old friends just enough that you're not forced into awkward social engagements to maintain the tentative friendship, and when you do have chance encounters, you can at least bring up whatever they've been posting about lately. Antisocial? Nope. Just social enough.