Developers will gravitate to what makes them money. If it was marketshare then more developers would be writing apps for Symbian than for iOS or Android.
Now that Android has exploded in that market, I expect most of those developers to migrate away from Symbian to Android if they haven't already.
One thing to remember is that we're talking about the entire population of software developers. Certain demographics (like HN readers) will show up more heavily on certain platforms (like iOS). However, the general trends will always mirror marketshare.