Foursquare has been taking VC meetings recently, looking into raising a Series D round of funding of between $50 million and $100 million according to multiple sources. In previous rounds, the company has brought in over $71.4 million in financing from Andreessen Horowitz, Spark Capital, Union Square Ventures and others.

From what we’re hearing the company seeks to flesh out this round at a pre-money valuation of between $700 million and $800 million, and that number has a few VC shops skeptical due to its growth trends, which have not gotten more inspiring after the company’s recent June redesign, according to multiple sources. Foursquare raised a $50 million round at a $600 million valuation in June of 2011 and still has plenty of money in the bank as far as we know.

Rumor has it that a few firms have eschewed the deal as priced, including Ignition Partners. We’ve also heard a couple of whispers that a less expensive round might surface more investor interest, at a $300 million – $400 million valuation pre-money.

Perhaps investors are having a hard time drumming up enthusiasm because the company is valued at just under half the market cap of its public competitor Yelp, which boasts 84 million unique monthly visitors and 33 million reviews. The company has 25 million registered users according to its website, and has 4 million monthly active users logging in through Facebook Connect according to AppData (the total number of monthly actives is not disclosed by the company). However CEO Dennis Crowley did confirm that the app had more daily active users than Yelp’s 7.2 million in an interview with Sarah Lacy in October.

Foursquare reported 15 million registered users in December 2011, 20 million in April 2012 and 25 million this past October: While it seems like the traditional user acquisition rate isn’t increasing, especially in the U.S, we’ve heard that growth is promising in other countries, such as Brazil and Turkey. We’ve also heard that a particular statistic representing online to offline behavior driven by the app is assuring, as 20% of users who search for a given place on Foursquare Explore actually end up checking into that place within 72 hours.

This statistic is key as the company focuses on refining its revenue model. When users search for a venue in the Explore tab, they see sponsored venue results reminiscent of Google AdWords or ads in Google Maps at the top of the page. The company is currently testing these Explore Promoted Updates with about 25 brand partners, including Best Buy and Old Navy, on a cost per acquisition (or check-in) basis. The company views the potential market for this feature as the over 1 million merchants on its platform, but ostensibly needs a larger target userbase for it to be effective. The Explore feature is presently seeing over 1 million queries from users a day.

Brand deals like its partnership with Amex also bring in cash, but investor Fred Wilson confirmed that the company did not have “a ton” of revenue yet this September. For comparison, Yelp is expecting its revenue to come in between $136.4 million and $136.9 million for all of 2012 and has over 35k paying merchant accounts.

Though a popular acquisition target because of its talented founder and 150-person team, mainstream adoption still seems to be a challenge for the darling of the New York startup scene: it’s difficult convincing potential new users that it’s a substitute for Yelp and already existing users that the product goes beyond checkins and badges.

The rumors of a new financing have been unconfirmed by Foursquare but a representative gave me the following statement on the company’s engagement numbers: “Our numbers have continued to go up, especially since we made Foursquare Explore available to everyone on the web. People use Foursquare over a million times daily to find where to go, and over 5 million times daily to share where they are.”

Word on the street is that the company will use the potential new cash for engineering resources and expansion, as it continues to hire and fill out the second floor of its New York office.

Additional reporting by Romain Dillet.