In the past couple of years — it seems like forever, really — the tech world has been fixed on finding how much the biggest social network, Facebook, is worth.

Right now, if you trust the valuations at SecondMarket, an online marketplace for trading with otherwise illiquid assets, Facebook is currently worth around $41 billion, Businessweek has learned from a source familiar with the matter.

Back in 2007, Facebook was said to be worth $15 billion; its valuation "dropped" to a "meager" $3.7 billion at recession's peak, but it has risen unstoppably ever since.

In September, the Financial Times said Facebook was worth $33 billion, which meant it was bigger than eBay, a publicly traded company that was then valued at $32 billion. Now, with a $41 billion valuation, Facebook is once again edging over eBay, which is currently valued at $39.3 billion.

Of course, all these numbers and valuations that are thrown around have one thing in common — they're not real. Yes, Facebook is one of the hottest technology companies around, and, yes, lately it's been showing solid business results. But the current valuation is less concerned with the business side of things — eBay's revenue, for example, is expected to top $9 billion this year — and more with expectations, which start somewhere at Facebook further expanding its huge user base and increasing ad sales, and end at more than 1 billion users and world dominance.

Unrealistic? Who knows. Risky? Definitely: one only needs to look back at MySpace and how quickly it fell from a social networking powerhouse to a has-been. You can trust the current Facebook valuations on SecondMarket, and if you're feeling really bullish about social networking and Facebook's future, feel free to raise the bar even higher. But have in mind that only after Facebook goes through with an IPO, the market will determine its value. Until then, its valuations are mostly speculation.