O'Reilly Conferences Brand name venture capital firms are all desperately trying to stuff cash in Foursquare cofounder Dennis Crowley's pockets.

One of the venture capitalists coveting Foursquare told us:

"Anybody who can [invest in Foursquare] at [a valuation] under $100 million is going to make money."

For most people, this $100 million valuation probably seems…crazy. After all, Foursquare is only a year old, doesn't even have a million users yet, and revenues are still a distant dream.

So: why?

Why are these investors -- Accel Partners, Andreessen Horowitz, and Khosla Ventures -- so eager to throw cash at Foursquare on such inflated terms?

We asked around, and came up with a couple answers.

For starters, get it out of your head that the reason these VCs want Foursquare so bad is that they think Foursquare is going to be bigger than Twitter some day. Also dismiss the idea that Foursquare's huge valuation has anything to do with these VCs believing Foursquare is going to crack the local advertising nut.

A much bigger factor is startup scarcity.

One VC source complained to us recently that these days there are "so few deals and so much money." The VCs trying to get Foursquare agree. From their perspective, there are very few consumer-facing startups with the ambition of Foursquare. Foursquare is the only deal to get. Meanwhile, there's a ton of VCs firms with piles of cash. The supply and demand equation skews in Foursquare's favor.

The other huge factor to remember is that investing in Foursquare is not necessarily a bet that Foursquare will gain mass consumer adoption.

Investing in Foursquare, is actually just a bet that any of the five following things will happen:

That some company -- Google? Yahoo? Facebook? -- will believe that Foursquare can gain mass consumer adoption.

That some company will buy or develop a Foursquare rival and that another company will come to believe it needs Foursquare to defend its market position.

That some company -- we're thinking of Yahoo here -- will decide that buying Foursquare for $100 million is a cheap way to demonstrate to its employees and investors that it is interested in making bets on startups that could gain mass consumer adoption.

That Foursquare will gain mass consumer adoption

And, most importantly: That someone,someday will come in and bet on any of the above happening, but at a higher price.

Don't miss: Yahoo Considers Buying Foursquare For ~$100 Million