your personal account on facebook to become your friend ? (So you search on facebook members of your site and request to become there friends)
Or use your facebook page and search them on facebook and invite them to become friend of(you) and the page of the site.
I don't do both.
I don't usually mix my personal and professional lives together to often. Unless I have a friend I know who might be interested.
I don't really add people I know online unless I've known them for awhile. And even then, I don't bring up online stuff unless they do it first.
I don't do both. I don't use FaceBook yet but I would keep a personal page and if members from the forum want to join they can.