i think this will answerer your question http://en.wikipedia.org/wiki/File:World_Wide_Smartphone_Sales.png
i would also point out thats just smartphone not tablets as far as i am aware android 100's of diffrent tablets , apple quite a few diffrent tablets ipad 1 ,2 ,3 mini etc and quiet a few sales
Symbian i goggled Symbian tablet could not find 1 just design concepts for them
so
Symbian make up a very small % of market less people that have that phone less time developers spend making apps for it less online documentation there is for it etc and a downwards spiral begins small amount of users small amount of demmand for developers lack of previous source code and community help makes harder for people that want to start in Symbian
but that is just most of the world in japan Symbian is huge but i cant read Japanese documentation so its worthless to me
if you want to develop symbian apps i belive the SDK's are free http://www.developer.nokia.com/Develop/Featured_Technologies/Symbian_C++/Tools/
but as you can see from there examples the latest example to be put up was done in 20 March 2012 and latest documentation added was in march 2012 as well look on android forum latest example 3 mins ago, last bit of documentation 20-30 seconds ago.
now i know its a bit different since most of the android is community driven and not on the official site but does make a difference when learning since 1 few dozen example other 100's of examples added each week
i wouldn't say Symbian is dead i would just say its been crushed by its bigger brothers with in the mobile development world mainly android (free highly maintained totaly open source both the SDK's and the os it self and can be made to work on nearly anything)
bit like dreamcast and playstation 1 didnt matter dreamcast was really equal to psx and also had extra functionality like internet browsing kids wanted the psx it sold in abundance dreamcast was then found few months later in the bargin bin of most supermarkets and the people that bought the dreamcast found they couldnt get half the amount of games for it that psx had