What I've learned from over a year-and-a-half of being on Facebook.

A simple list in no particular order:
1. Facebook is great for reconnecting with people you've lost touch with over the decades.
Results can be wonderful--or sometimes quite bittersweet.
2. Some people friend others on Facebook for sincere reasons; others do it to rack
up their numbers-of-friends (just like in real life).  Glad that Mark Zuckerberg hasn't thought
of Frequent Friending Benefits quite yet.
3. If you had only a slight acquaintanceship with certain people in the normal world, it won't be
any different after you've friended them on Facebook.  Be prepared to post something to someone's wall/page and receive no response whatsoever (exception: when you're posting something complimentary about them).
4. Facebook is something I highly recommend for finding groups for just about anything one may be interested in.
5. As film critic/writer Karina Longworth said in a highly readable article in the recent issue of L.A. WEEKLY, Facebook is a place where your data is given to corporate advertisers to harvest.  Be aware of that when joining so you can control what you choose to share.
6. One person posting links can be another person's "spam."  I found that out (rather bluntly) earlier this year when I was unfriended by an Orange County poet.  But the other person could have easily skipped over the links he didn't want to read.
7. As with the real world, you won't please all the people all the time on Facebook.  Don't stress out
over it.