Facebook has become something of a "Third Place," in this respect.

http://en.wikipedia.org/wiki/Third_place

if people don't reply to your emails, maybe that says a lot about how much they care about you.

Exactly? I'm talking about a situation where we both care exactly at a "casual Facebook friend" level. Like we would meet up if we were in the same town at the same time, but we aren't writing emails back and forth about what has happened lately. And that is great, IMHO.

Or maybe it says nothing about that -- but more about their email patterns, work stress, guilt, etc.