Is it possible that someone you emailed (or that emailed you) in regards to that guild had their email highjacked?
I get hit up by Facebook when people let FB pull in their contact list from their email. Then FB solicits me to join so I can be friends with them. The thing is, I have a FB account through another email, so I can tell when the "invitation" is phishing.
I have found that when searching other commercial sites that leave cookies on my computer, the ads that pop up here pertain to where I've searched/shopped. I don't see that as this forum's fault, though, it's all part of how other sites track where you go and keep hounding you with advertising.
For what it's worth, I agree that it's important to keep one's data private. This data mining is a source of big income for websites, and I don't like being used. But I have never felt used by this board.