Hold on to those employees for dear life, Google, because Facebook just keeps getting more powerful by the second.

The world’s largest social network had over 151 million unique U.S. visitors in October, according to comScore data, an increase of 55 percent from the 97 million uniques tracked in October 2009.

And who’s surprised?

Facebook registered its 500 millionth member sometime over the summer and 50% of active users log on to the site on any given day.

Increase in mobile usage, especially of smartphones like iPhone and Android, probably helped Facebook’s phenomenal growth this year. Over 200 million people access the network via mobile platforms. A year ago, only 65 million people used Facebook on mobile.

Additionally, Zuckerberg attributes much of the site’s dramatic growth in the past year to the rapid rise of social gaming led by Zynga, Playdom, Playfish and Crowdstar, all developers on the Facebook platform. The same has been true on the iPhone and even on the PC, he noted; gaming is always the first vertical to tip.

So with MySpace out of the picture and Twitter growing comfortably on its own turf, Google, the king of search, is left looking like Facebook’s primary competitor. The tension between the two only looks more intense when one considers how close Microsoft and Facebook have become as of late.

Facebook actually surpassed Google, in terms of visitors, in March of this year, according to data from analytics company Hitwise. (The graph in that article speaks volumes.)

Basically, it comes down to social. Zuckerberg predicts that we are advancing toward a time where every aspect of the Web will be social, and the companies that don’t embrace social will fail. Of course he’s biased, but the Web seems to agree with him. The open social graph and Instant Personalization launched less than a year ago, flooding websites with Like buttons, and already the features are permanent fixtures of early every important service on the Web.