- Would clutter my already-cluttered inbox
- Wouldn't be invited to anything
- Takes more mental energy; email etiquette is much more formal than Facebook in my experience
- Facebook has profiles you can go off of; places where you can quickly and easily see how people interact with others, what their interests are, what their priorities are, how they use Facebook. Email starts you from scratch and makes for LOTS of small talk to get the exact same information
The first reason is the main one; even when you do mass emailing, it feels much more individualized. When used well, Facebook wall conversations/private message conversations can flow exactly like group discussions (in person or on a forum). From my experience, email can't even come close to replicating that.
Also I am curious to the answer to @highlander's question -- since IME Facebook's privacy settings are very easy to figure out, and very reliable when you get them right and when you learn to expect revision.
I never understood people who have a lot of Facebook friends they MET on Facebook and never spoke to irl -- because as I understand it (and I'd like to think I understand it well), that's not the point of Facebook.