Re: the hockey stick--m..maybe? This is entirely anecdotal evidence, but the "San Francisco" network spreads pretty far across the entire Bay Area (which is pretty quick to adopt services like this, obviously), and you can still find yourself within the top 1,000 checking in once or twice per week (e.g. using the service). Back of the napkin with those figures, I'd estimate regular usership between 10k-20k. Granted they're branching out to new areas and new devices all the time, but I'd still say they're just getting to the blade of that hockey stick.