The main caveat here is the obvious one - that this is only tracking browser use on phones, not overall phone use. In other words, the significant proportion of the phone user market who don't actually browse web sites on their phone aren't counted at all. Don't confuse StatCounter with sales or marketshare!
Here's the last two years of stats for mobile OS, anyway:
What's interesting here is the way the Symbian curve doesn't dip at all after February 2011 and yet falls away sharply around April 2012, matched exactly by a rise in the value given to 'Series 40'. It's clear that, prior to this date, 'Series 40' use wasn't being counted on its own at all, and that any S40 browser use was being assigned (erroneously) to the Symbian figure.
This is backed up by a statement from StatCounter here:
In April 2012, we performed a Nokia OS detection update. Prior to this, Nokia devices (including some S40 devices) had been grouped largely under Symbian OS. With assistance from Nokia and a number of individual users, this error was corrected on 23rd April 2012 and from that date all Nokia devices are now tracked as Symbian or S40 or Meego as appropriate.
Assuming that the real 'Series 40' curve should be gently rising to the current (correct) figure of 15%, then we can assume that the Symbian curve should have been a) lower in the first place by up to 10% and b) falling gently, to its current correct figure of about 13%.
13% isn't that high, but all things being equal I'd have expected the installed base of 200 million or so Symbian users to have produced a higher showing than this. However, remember what I said earlier - this is just active browsing usage and I think we can assume that many people round the world using classic devices like the E71, 5800 and even the N8, all undoubtedly still in use as 'smart' phone, aren't actually using them to browse much. Especially so in comparison to iOS and Android phones, which place their faster browsing experience far more up front and centre in their users' minds.
That the renewed Series 40 should be figuring more in the stats is quite right, mind you, with the newer S40 handsets offering far more Internet functionality than before, and sales seem to be going very well in developing markets.
Lies, damn lies and statistics, I know, but I just love poring over this stuff...! Comments welcome, and do go try out StatCounter for yourself. Any particular charts you want to produce and share?