Location based social network Foursquare is the darling of the media's eye these days and appears to be growing very fast. Rumors are growing louder that Netscape co-founder Marc Andreesen's venture capital fund is about to invest in the startup and the company now reports that it sees "100,000 new users every 10 days."

Is old-school local social network Yelp scared? No, says Eric Singley, Yelp's Director of Consumer and Mobile Products. Singley told us he finds Foursquare's user numbers "hard to quantify" and said "they don't blow my mind." Foursquare may be fun, he says, but Yelp remains the best place to find good restaurants and bars to patronize. Yelp is also much bigger than Foursquare. Tonight the company is making a number of mobile announcements that seem aimed directly at defending Yelp's crown.

Yelp Mobile is Already Big

Starting tonight, local businesses that have created free accounts and claimed their listings on Yelp will receive additional analytics specific to mobile user traffic: the number of times they've shown up in mobile search, the number of times mobile users have clicked to call them by telephone and the number of times users have clicked for directions to their business.

Yelp says that 2 million unique users searched on a mobile Yelp app last month, performing 27% of the total searches the company saw. The entire Foursquare network claims 1.8 million users. Yelp said this Spring that it sees 31 million unique visitors to its website every month.

That means Yelp is probably at least 15 times as big as Foursquare. The tech press is fixated on Foursquare (ourselves included) because it is more social networky, more innovative and may be significantly faster growing. It's only 15 months old. It's also a lot less useful than Yelp is.

Founded by an ex-PayPal crew more than five years ago, Yelp has raised about $30 million in venture funding itself. Two and a half years ago it had a rumored valuation that was already two and a half times what Foursquare's reported valuation is today.

Possibly as soon as next month, Singley says Yelp will release an Android app with check-ins, a feature clearly modeled after upstarts like Foursquare but that Yelp says is proving successful with its users as well.

Foursquare is very interesting, as a multi-purpose location platform it may prove a very disruptive technology. Maybe. But it will be hard pressed to turn the tables between business owners and consumers the way Yelp has in this country and it remains a fraction of the size as a business, despite the hype.

Business owners all around the United States will begin getting emails tonight showing them just how important Yelp's own mobile applications are to their bottom line. Foursquare loyalty programs might be big someday, but Yelp mobile numbers are likely to make a big impression right away.