I have mellowed with a lot of my earlier criticisms of Facebook use, but I still consider these four commandments to be self-evident and inviolate:
Thou Shalt Not Post a Dozen Items Each Day
If you've found an interesting news article or you'd like everyone to read your latest blog post, go ahead and post it! But please don't spam me with your ideology by posting several of them a day. This amounts to advertising and becomes no better than a usenet newsgroup, and is one of the main reasons why I don't go to usenet newsgroups anymore.
Thou Shalt Not Write Obnoxious Status Updates
There's nothing more off-putting than getting a bunch of status updates written for the sole purpose of offending you or shocking you.
Thou Shalt Not Invite Others To Join Fanclubs Devoted to Thyself
If you actually have an organization, business, or band that you think people might be interested in, that's fine. But it's SO cheesy to receive an invitation from Mr. X to join the "We Love Mr. X Fanclub," and I think such invitations say a lot of unflattering things about the senders.
Thou Shalt Not Send Dramatic Emails About The Level of Drama in Facebook
It's PARTICULARLY annoying when the emails say "...and I'm LEAVING, and my REAL friends will know where to find me," and then two months later the person has started a new account and is asking everybody to be friends again.