I plan to decline any missionary friend requests. And send a pre-emptive warning to my FB friends that they may be contacted and to feel free to de-friend me if they want.
Why waste such an opportunity? It would give you a great chance to ask them some questions back. Invite your mormon friends over for a backyard BBQ and keg.
Make them back down while you hold your head high.
One more problem with FB..they suggest "Friends" for me that are all ready on my list. Weird, I know. But I always check to make sure they didn't drop off for some reason. Yup. Still there. There are many quirks in FB that keep me scratching my head. So much technology and so little oversight! If I get missionary requests, I'll ignore them.