Foursquare has announced that it’s closed a new $20m Series B round of funding with Union Square Ventures, O’Reilly AlphaTech Ventures and new partner, Andreessen Horowitz.

"The two big names behind Andreessen Horowitz – Marc Andreessen and Ben Horowitz – are each legends in Silicon Valley," says Foursquare Founder and CEO Dennis Crowley. "They know better than anyone how to transform startups into successful organizations. As we continue to rapidly expand to take advantage of the opportunities in front of us, Ben and Marc’s expertise in growing companies will be invaluable."

Horowitz says on his blog that his company invested in Foursquare for three main reasons: a great Founder/CEO, a killer product, and a gigantic market.

There’s no question that the market for what Foursquare offers is heating up. However, so is the competition. Clearly Foursquare is leading the way right now, at least in name recognition and press coverage, but the market is going to continue to be flooded with various check-in offerings, and it may be hard for one service to truly dominate.

Also consider that Twitter now has its Places feature in place, and Facebook will be coming out with its own location features any time. It will be interesting to see how these impact the future of services like Foursquare.