At first my self imposed rule was that I would only accept invitations from people that I really knew.  Then I started viewing there lists of friends and saw other people that I knew but had not invited me. So I started inviting them to become my friends.  Recently, I found myself accepting an invitation from some I did not know but who wrote saying she knew one of my good friends very well. I guess, soon I will be contacting people I do not even know.