Serious Business founders Siqi Chen and Alex Le created the Facebook application Friends For Sale, which has over 550,000 daily active users. On Friday, the pair announced they'd landed $4 million in a first round of funding from Lightspeed Venture Partners. The investment gave the company a "healthy double-digit" valuation, according to Inside Facebook's Justin Smith. (For Chen and Le's sake, we hope there are another six digits after those two.) "We're already profitable, and the investment is going to help us get a head start," Le told Smith. Impressive, if true. Smith recently compiled a list of the CPMs, or cost to display an ad 1,000 times, Facebook developers earn on their applications. The average came in at $0.92. One app earns as much as $4.78 per thousand views and another just $.04.