It might not be Facebook itself, but the overwhelming firehose of constant updates from your friends and family, combined with the knowledge that you CANNOT (should not?) use it for venting frustration or anger at the times when you most want to -- because the people you are likely wanting to vent about are your friends or family. ;-) (Well, that and the tinfoil-hat aspect of not wanting to post anything For Posterity that isn't completely sanitized.)