Many of the apps are like digital trading-card games. You send little pictures or whatever around. There's no harm in ignoring them. There are some that are more game-like. My friend got me into one called Knighthood, in which you build a castle and then try to take over others' territory--it's more like a real game. But if people can't handle you refusing to play, then they're a wee bit oversensitive about Facebook.

I don't know about the risk factor. I've wondered myself.