I think you’re missing the point of the facebook community - you don’t NEED to keep in touch with them, because you know where to find them if you want to.
I look at it this way. Back in the day (if you will), we had a community in the physical space around us, people you’d wave hello to, but not much more. People we saw at the grocery store, post office, bakery, etc. Our kids’ teachers, their friends’ parents, school crossing guards, etc. Now that we’re (a) a lot more mobile and (b) a lot more isolated in our cars and big box stores, we often don’t make such connections in our communities, and can even go days and days without talking to a real person outside of our immediate family and co-workers.
These people - the shopkeepers and crossing guards - are not people you would consider your “friends” or invite to weddings (or really make any effort to keep in touch with at all). You just see them every day, say hi, and go on your way.
I think it’s really important to have relationships like that, incidental ones that don’t take proactive effort, just people whose face or name you see and have a moment’s thought of “hey, I’m glad that person’s around” before you move on to another thought. People who you would never invite over for dinner or out for a drink or really make an effort to connect with. The vast majority of my Facebook friends fit that description - they are friendships that demand nothing more than what I feel like putting into them.
Old schoolmates are the ideal people to have these sorts of relationships with. I have many such people. Just because I don’t really see them any more (because we no longer have anything in common) doesn’t mean I don’t value them somehow; on Facebook I get to have them as my friend without having to listen to them drone on about their perfect children or thrilling job or mortgage rates or whatever.
I consider these to be just like neighbours in physical communities who you don’t know any better than to say hello to on the way to work.
This:
I don’t understand. It’s a common meme, that Facebook is used so people can keep all their friends informed of the most minute detail of their lives. Neither I nor any of my friends use Facebook in this way. We mostly use it as contact management software (i.e. to keep in touch with people with a million e-mail addresses and phone numbers), to keep folks posted on major happenings (e.g. “come to my party on Friday” or “I’m away on holiday for two weeks which is why I’m not answering your phone call”), and to pass time (just like on the 'Dope). There may be people who write down every detail of their lives but that is certainly not the only function of Facebook.
