I read somewhere that Foursquare has grown by over 3000% in the last year.
Sound simpressive until you think about how few users it had in the first place.
One of my co-workers was telling me that by cross referencing people's Twitter posts with their Foursquare pages you can tell what prospective clients they are off to see.
I think this has to do with over capacity too.. I use Twitter loads especially on my phone and it tends to not refresh the feed / send DMs etc when over capacity.