I play both here and on FB.
There are a few software differences - some of Java bells and whistles are not allowed in FB apps. You can't, for instance, click backwards and forwards through the map to graphically see the ebb and flow of the game. You do get the IM areas, but they don't automatically scroll to the last entry. Like I said, just missing a few bells and whistles. The game is very playable.
And yes you can get to someone else's FB Id, but, unless they're idiots and actively turned off all privacy settings, all you can do is see the tiny public photo (me on my new motorbike!) and send a friend request. No email addresses, no group memberships, no status updates, no favourite books, ...
The quality and number of players is higher here than in FB. But then, you have to actively find this place.