Lisa - I have wondered this also - when people try to add me that I don't know, and we have no mutual friends either, I just leave it sitting there on the basis that if they didn't know me in the first place and were just trawling to get extra 'friends' then they won't even remember that they asked me.
On the other hand - for all I know - they might get a big red flag coming up saying that I'm ignoring them - but I can't imagine that's the case...
