The email you never received:

Subject: New friend request from Mark

Mark wants to be added to your Friendster friends list. By becoming friends with Mark, you will be able to send Mark messages and connect with his friends.

He would have invited you a long time ago, but, c'mon, "Friendster"? It just sounds like another internet fad, the kind of thing sent to you by the same sort of people who tell you that Bill Gates will send you $1000 for forwarding on this email. However, Mark and his wife both like meeting new people, and if Friendster helps, then why not? But he would rather not risk annoying you by putting an invitation in your email inbox.

So if you are doing the Friendster thing, drop him an email or leave a comment and he'll send you a proper invitation.