Once (a couple years ago) I'd see foursquare stickers and offers of discounts for checking in and people would use it, but I don't know any of those 20M anymore - within my world (SF, SOMA) foursquare seems to have dropped off the map.
Right, that's my thinking... based on how much I used to hear about Foursquare years ago, I would think they'd have more users by now if they're on a path to success.
When I looked at some business reviews on Foursquare (while traveling— was in Washington, DC area) it was disconcerting how a large majority of reviews seemed to be from 2009-10.
"Only" 20 million users? "Only"? The HN commentary is an interesting place these days.