A while ago we got a lot of spammers and I believe one of them sent a LOT of people (if not everyone apart from the mods and admins) a private message thanking you for being a member of the forum, and asking you to take a survey.
This was a spam bot that sent these, and the bot has since been deleted. I do not know for sure if this deleted the messages for people or not, too.
NEEDLESS TO SAY, DO NOT CLICK ON THAT LINK!
I've done a little bit to increase the anti-bot junk on this forum, though I am not super knowledgeable there. For the time being I have also disabled private messaging.
Sorry for the inconvenience.