College is running an ad campaign, one of your co-workers saw it (consciously or unconsciously) earlier in the day, and that is what lead to the college coming up in conversation?
Your friend could have searched for the college before or after your discussion, or maybe even your friend had just been looking up old college friends on Facebook lately and that was enough to match up that ad and you. Not that I wouldn't put it past Facebook to do something like this, but it will be hard to prove from anecdotes because, ironically, Facebook captures so much other data that plausibly could reveal the same connections.
Also, let's say what you are saying is true. How come ad decides to show up 30mins - 60 mins after conversation takes place?