"Only your friends can add you into groups."

And how many of your "friends" are ACTUALLY your "friends"? As much as it was once fun to reconnect with somebody I shared an elementary-school classroom with, that's a connection I'll need to revisit if they're given the power to latch my profile onto whatever cause they want ... implying my support of that movement.

Facebook seems to have created Groups as their implementation of Twitter Lists. I can create a list and call it whatever I choose ... adding whomever I choose to it. But my sense is that the perception of a Facebook Group is different than a Twitter List because Facebook already has/had the concept of groups, and they DID require opt-in.