I think the OP is talking about being able to notify a user when someone has used their name in a thread so that they can follow it, similar to Facebook, Twitter, and DeviantArt (who has a tagging system now too). He wasn't suggesting to be able to tag people here on E with their Facebook profile.
However, to touch on your concern, I am part of a FB group for the Wyrmrest Accord server on World of Warcraft. There are tons of people on there that don't want their real life mixed with their WoW persona, so they chose to make a separate Facebook profile for that. I've done this before and just used my main in Chrome and my alt in Firefox so I didn't have to log in and out all the time.