I've got a the facebookconnect plugin - and the memberprofiles module set up - both work fine.
however, I about 2000 users who set up their accounts before I added facebookconnect.
It would be awesome if they could log in, and 'add' facebook connect to their existing account, so that from then onwards, they can log in to the same account either using their SS login, or their facebook login...
I think the module should work fine retroactively. One issue will be if the email of their Facebook != email they used to login to SS then you would perhaps need some smart account merging ability. It's a pretty simple module, the main part you would need to test is the onBeforeInit logic - https://github.com/willrossi/silverstripe-facebookconnect/blob/master/code/FacebookConnect.php#L209 as that handles all the connection between the SS data model and Facebook.