You need to have your own IDs IMHO, so that people can associate multiple accounts (ex., both Facebook and Twitter). Could you go with something simple, like a

You need to have your own IDs IMHO, so that people can associate multiple accounts (ex., both Facebook and Twitter). Could you go with something simple, like a timestamp with microseconds or a timestamp plus a random number? If you don't want to allow that, why not just to fb_{facebook_id, tw_{twitter_id}, etc.?