I've never heard of them either...
I guess I'm too old (in the 35-40 years-old range).
Well, actually, I heard a friend mention "Foursquare" in relation to a dinner date, though I never quite understood what he meant. He said something about "checking in at your destination" and I asked, "why?" to which he responded with a blank stare.
He's not so much into the Web-Two-Point-Oh-Social-Networking-And-Badgers clan, as he is into the Lets-Get-Rich-Quick-By-Making-A-Web-Site-That-Exploits-All-The-Latest-Buzz-So-That-It'll-Be-Eventually-Bought-By-Google-Yay! crowd.