I think you're reading too much into it. I barely use facebook and have a couple hundred friends. I'll accept an invite from anyone I know and like, regardless of how well I know them. Do that for a decade, or however long FB's been around, and you build up quite a few.
It's very tempting to friend anyone. But because of Facebook's tendency to then cross pollinate posts via comments etc. I'm more reluctant to do so (privacy of others). I post to Facebook almost as if I'd post in public. But it's different for everyone. I know there are settings and ways to make circles, but it seems a faff.
