a wiki-format site for information about startups, is giving users a chance to put some money behind their opinions on which startups will be hits and which will fail. Of course, the money is fake, but the is a fun addition to the site.

The new feature — which just launched in its public “alpha” test — is basically a fantasy stock exchange for startups. There are 50 companies listed on the exchange right now, and registered participants are given a fake $100,000 to invest as they choose. Buying and selling proceeds as it would in a normal stock market, and the fluctuating stock prices provide a broad reflection of the community’s view of a company.

Since each company has a million shares, you can even estimate a valuation by multiplying the stock price by 1 million. So, as of about 11:30 Monday evening, the Startup Exchange valued professional networking site LinkedIn at $864 million, and build-your-own-social-network startup Ning at $570 million.

(The Ning valuation is pretty close to the $560 million post-money valuation in its latest funding round, while LinkedIn’s falls a bit short of its recent $1 billion valuation.)

Of course, I’d put about as much faith in these valuations as I’d put in YouNoodle’s startup predictor — less, even, since this is explicitly just a gauge of public opinion. But it will be interesting to see how companies rise and fall. (More statups will be added over time.)

It’s also a smart improvement to the TradeVibes site. Mill River Labs, the company that built TradeVibes, has been landed partnerships with news sites like Mashable and The Inquisitr to help raise its visibility. But it’s fun social features like the Startup Exchange that will keep users coming back, and perhaps help TradeVibes snag some of the traffic from CrunchBase, its more famous competitor.