My kids (DS22 and DH 19) both are friends on FB with their Grandmother, Uncles and myself. They aren't uncomfortable with how they live their lives and have no problem with family who have facebook being friends with them. As far as what their friends post to them ...well, my kids have decent enough friends and though sometimes what they say isn't always the most appropriate it is outside my kids control. I think that grand parents who are savvy enough to have a facebook account understand this.