Is there a way to cancel a friend invite I've sent?

The cancellation button will not show up for any pending invitations until they're two weeks old.  After the two week period, you should be able to cancel an invite.  Here's how:

1.  Under Messages at the top of the page in gray, click inbox.
2.  On the left, click the "Invites Sent" link.
3.  Click "Cancel Invite" below the invitation you would like to cancel.