"A friend of a friend" is enough for me to take someone's call if they're asking for a personal or professional favor. Lord knows I've been on the receiving end of enough of those gifts.

The thing I'm noticing lately is most of my LinkedIn invites nowadays are from people that I have never heard of, but have as mutual friends. It's almost as if that space has gotten big enough to let more folks in. Once you have 500+ friends, what's 501+? Though the impact of being so open is sharing less with your friends.