But how many of these reported users are actually one person logging in from three different devices, as the two commentators above and I also tend to do. And this isn't because of not having another device to access the social networks from at all, it's mostly because it's easier to reply to Tweets on my smartphone while my PC is churning through graphics manipulation or whatever the daily workload of your PC is...
I tend to think this "smartphone bringing social media access to the previously underserviced market" line is a bit of a dreamland. I think most people who access social media on smartphones have at least one other way of connecting to the service as well, the phone just becomes the most convenient. It fits better - the phone is the custom-made comms tool the PC has other uses. People who don't have access via the PC aren't going to go out and get a smartphone just to have FB and Twitter access, and since they aren't FB and Twitter users already from PC experience it isn't going to be the reason they've got the smartphone, but rather just an interesting new capability that the phone now has which they may or may not choose or want to use.
Anyway interested to see the stats.