My initial activity on Facebook lent me to accept some random invite requests or random invites being sent out from my gmail contacts. So I decided to clean my "friends list" a bit and clear out the randoms and professional contacts I would prefer to limit to my Linkedin account (i.e. recruiters, headhunters).
For me there isn't an exact criteria since I do believe Facebook is useful for some professional contacts, and keeping in touch with acquaintances. I've been eye-balling my "friends" directory, thinking about how I know someone (or don't know), and then removing those I would prefer to know through Linkedin or email. I've removed over 40 so far.
Hopefully this will keep my small world a little more manageable.