The largest mobile (web) platform is ... WebKit

• From: http://en.wikipedia.org/wiki/WebKit

• As of November 2012 it has the most market share of any layout engine at over 40% of the browser market share—ahead of both the Trident engine used by Internet Explorer, and the Gecko engine used by Firefox.

In face, you can install WebKit-based browsers on any OS platforms like Chrome and Safari on Windows, Mac, Linux, iOS, Android, ...

• It is also used as the basis for the experimental browser included with the Amazon Kindle e-book reader, as well as the default browser in the Apple iOS, Android, BlackBerry 10, and Tizen mobile operating systems. WebKit's C++ API provides a set of classes to display web content in windows, and implements browser features such as following links when clicked by the user, managing a back-forward list, and managing a history of pages recently visited. 

So more and more platforms will develop their browsers based on WebKit, it a trend.

• WebKit's HTML and JavaScript code originally began as a fork of the KHTML and KJS libraries from KDE,[6] and has now been further developed by individuals from KDE, Apple, Nokia, Google, Bitstream, RIM, Igalia, and others. OS X, Windows, GNU/Linux, and some other Unix-like operating systems are supported by the project.

Apple, Google and Microsoft rely on WebKit for developing their browser, WebKit is obviously the largest "Mobile Web" platform. It has chance to become the largest "Mobile" platform.

• As of the first half of 2010: WebKit-based browser, 350 million.
• As of February 2012, WebKit's market share at 36%.
• Total use of WebKit (Chrome, Safari, and smartphones/tablets) will surpass 50% by the end of 2012.

Which mobile platform do you target on developing mobile apps? I select WebKit!