Zynga, the maker of FarmVille, dominates Facebook games with more than 275 million users for its social games. But mobile gaming is another story.

If Zynga can take a big stake in the fast-growing mobile games segment, then it will diversify beyond Facebook and give investors another reason to get excited. Yesterday, the Wall Street Journal reported that Zynga was raising a $250 million round of funding that valued the company at $7 billion to $9 billion.

To be more than an also-ran in mobile, Zynga is starting to invest heavily. Today, it launched the Android mobile device version of its Words With Friends game, the popular iPhone title Zynga recently acquired with the purchase of Newtoy.

The game is innovative in that it allows friends on different phone platforms to play against each other, so an Android player could challenge an iPhone player.

The Scrabble-like word game launched on the Android Market this morning. The game lets users play with up to 20 of their friends simultaneously, engage in in-game chat, and receive push notifications about when to take a turn.

The Android version is available as a free download and it has an ad-free premium version coming out later this year. Zynga says that Words with Friends is the most popular multiplayer word game on the iPhone, iPad, and iPod Touch.

Words With Friends can be played across more than 50 Android devices today. David Ko, senior vice president for mobile at Zynga, said the company’s goal is to make its games available anytime on any device.

Zynga launched its first Android game, Zynga Poker, in December. Rivals include DeNA, Glu Mobile, and a variety of other mobile game companies.