I had originally signed up with StackExchange (back when it was only StackOverflow) with OpenID, since that was the cool--and in case of SO, the only--thing to do. As time has passed, so has the coolness of OpenID. Now, on most sites, you either sign in with Twitter or Facebook or an old fashioned local account system. In fact, StackExchange is now the only network for which I use my OpenID. Everyone else has moved on.

I know StackExchange sites let you sign in using a multitude of accounts including Facebook, but my question is: How do I associate my Facebook account with my existing StackExchange account and sign in using Facebook from there on?

I don't want to create a new username. I don't want to lose my rep points.