I've spent the morning playing with and writing about RockMelt, the social Web browser.

RockMelt hooks into Facebook Platform to let users browse content online and share it with their Facebook friends and tweet to Twitter.

Users can also integrate RSS feeds. Pretty much anything anyone wants to do to read or share content can be done in RockMelt without forcing users to open up multiple browser tabs to share content on Facebook or Twitter.

I'm a heavy Google user. I can do a search in the search box, preview a Google result and click on it:

If I like a search result I find on Google.com, I can share it with friends by easily clicking the share button:

Boom, all of that happened without me having to bounce to my Facebook page.

So you have in RockMelt is the ability to search Google and share on Facebook. This is very powerful stuff. Both Google and Facebook win.

But if Facebook were to buy RockMelt, it could theoretically swap out Google as the default search for Bing and really ding Google.

I mean, if social search is the Next Big Thing, what better way for Facebook to do this than own a browser and let users share information through it, both on the desktop, and hopefully soon, mobile?

So if I'm Facebook, I'm definitely considering buying RockMelt before it gets too big and too expensive and wielding it as a sharp ax versus Google.

The glaring irony of all this is Facebook could do this leveraging a product built from Google's Chromium work.

Though of course Chromium is based on Apple's WebKit, so what we have in RockMelt is just the latest of progressive iteration on Web browsers.

If I'm Google, I wouldn't take this threat lightly. More eyeballs are spending more time on Facebook, which in 2010 really ramped up its online ad capabilities.

comScore just found that Facebook led all online publishers with 23.1 percent market share in Q3. Facebook's market share has increased 13.9 percentage points from 9.2 percent in Q3 2009.

That's a huge threat to Google and it's $2.5 billion display ad run-rate. Armed with a browser as its users' new window to the Web, Facebook could do some serious damage.