Someone who either doesn't have many contacts on MSN
or simply cant be bothered to start a conversation with anyone despite there being many people online but no one is speaking to them.
To attract attention they will sign out for a couple of seconds then sign back in again, trying to draw attention to themselves so that people will talk to them.
Person 1: OMG that Jonathan has signed in so many times this past hour after being offline for like a second!
Person 2: He's an MSN-loner, he's just trying to get attention.