In the past few days, I've been getting Facebook invitations from people whose names I don't recognize, names that seem generated like Jessica Gentleheart or Elenor Goldriver. When I look them up, they usually lack pictures of themselves or have little or no friends. I think Facebook will have to start modelling user activity patterns to curb abusive behavior intelligently.
If you know me, I would appreciate a few words in invitations to help me recognize you, hopefully information that is not publically known like shared experiences or events. One major benefit of building social networks this way is that it makes phishing difficult. After all, anyone can masquerade as anyone on Facebook. All one needs is a name and photo to get started and, even if the real person shows up, it'll take time and effort to clean up the mess with little risk to the pretender.