Facebook has overtaken eBay's estimated worth of $39.3bn to become the third largest US internet business.
That puts Facebook firmly in third place behind Amazon, worth $74.4bn and Google, valued at $192.9bn.
Analysts said the majority of Americans spend a great deal of their online time on Facebook. The company's valuation highlights growing investor interest in social media firms.
SecondMarket does not release trading data publicly, but Bloomberg cited sources as saying Facebook's value on SecondMarket has more than tripled in the past year.
Facebook makes most of its money through advertising but, unlike most competitors, it enables customers to craft ad campaigns to reach specific audiences.