My wife and I both sync our phones to the same iTunes account, and have no problems at all. Paid apps (and any other apps) can be put on any device sync'd to that account.
One thing you have to watch is sync'ing apps that are on one phone that you don't want on the other one. So if you install one on yours and she doesn't want it on hers, you have to make sure you un-check that particular app to be sync'd when her phone is plugged in. Luckily it does remember your previous app sync preferences each time either phone is connected.
I'm not totally sure about the FB thing. My wife doesn't do FB anymore, so it's only on mine. But I don't think FB stores any data on your phone, you're just using the app to access what they store on their servers. It's just an interface alternate from the actual website. I know both of us have apps like a restaurant tip calculator, and a baby diary app, and the data stored on each phone is NOT transferred to the other phone.
Actually, when you sync the phone, a backup is made for each device which contains the data. The data itself is not synced or is even available through iTunes. It's only the apps themselves that are synced.