According to Foursquare's co-founder Dennis Crowley, as quoted by TechCrunch, the service saw 5 million checkins per day as of Feb. 2013. But that was the same as per-day checkins as the previous year. Flat growth to me, especially in their space.
The rest, and a fairly critical piece of my statement, about checkins moving to Facebook and Path I admit wholeheartedly was anecdotal and speculative.