Facebook Axiom #1: A relationship is not official until you change your relationship status.
Corollary: Changing you relationship status back to "single" will elicit multiple comments of sympathy and people wanting to know what happened. Be prepared to explain.
Facebook Axiom #2: The Cycle of Action on Facebook is as follows: Facebook designs a new interface. People complain en masse about it. Eventually, people figure out how the new interface works, and the compaints die down. Six months later, Facebooks redesigns the interface again.
Facebook Axiom #3: People will discuss with you during live conversation things which you post on Facebook.
Corollary: Only post those things you wish to discuss with people.
Facebook Axiom #4: People may have opinions about the pictures you post which are different than yours. Exercise caution!
More to come as I think of them!