That's kind of disturbing. I've been using Messenger for Desktop for a while now, and I thought it was an official Facebook product (just checked again and there's a disclaimer saying it's unofficial, but I don't remember it being there when it launched).

Yeah, while I'm against services going after third-party client developers 99% of the time, this isn't one of them. I hope he gets nailed to the wall for this, because he's creating actual confusion, which is the very purpose of trademark law.