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Twitter is worth $9 billion, based on reports that BlackRock Inc. (NYSE: BLK) is willing to buy $80 million of stock in the short message social network at that astronomical valuation. The figure shows that valuing large Web 2.0 firms remains based on perceived momentum as much as anything else.

The lineup of recent, new age Web companies values them at this based on market cap — Facebook Inc. (NASDAQ: FB) at $68 billion, LinkedIn Corp. (NYSE: LNKD) at $13 billion, Zynga Inc. (NASDAQ: ZNGA) at $2 billion and Groupon Inc. (NASDAQ: GRPN) at $3.6 billion.

Sources questioned by the media say that Twitter’s revenue in 2012 was $350 million, and that the figure could reach $1 billion this year. But that forecast also has been pegged as low as $600 million.

Zynga could have sales of $1.3 billion this year, but its model is thought to be broken by most analysts. There is too much competition for online games, and Zynga’s reliance of Facebook , which has eroded, hurts it.

Groupon is also a victim of the belief that it has no real defense against competitors, although its revenue should be more than $2 billion this year.

LinkedIn is the only one of the companies, excluding Facebook, which is almost universally seen as having a business model that it can defend. The company makes money, has three divisions that bring in substantial revenue, and sales that should be higher than $1 billion this year. LinkedIn also has 200 million registered users, which means that if it can do as well mining them as it has in the past, sales and profits should each expand nicely.

Twitter has to deal with the “moat” problem, which is a staple of the education of every business school student. How easy will it be for other firms to compete with Twitter. Based on its valuation, not very easy at all. Twitter, like some small number of companies that are part of the new age Internet, can claim it has no competition. It has not turned that advantage into much revenue. Groupon and Zynga can say that they have made their models into revenue engines. But see where that has gotten them.