I have noticed that this has not been updated in quite some time and just making this post to get some notice to it because i would always check it out to see if someone is online and then go ahead and pm them that way.
Basically, when this feature was first created, it was making about 50 calls to XBL every few minutes to see if someone was online. Since the NHL Team, it was now making about 500-600 calls every few minutes. Quite frankly we couldn't keep up with the calls needed and i feared this service was resulting in some of the hosting problems lately.