Clouds are gathering over Silicon Valley’s consumer internet companies. The sale of social networking company Bebo (our coverage) comes at a time when private investors are changing their tune. They’re no longer pumping money into start-ups at the same huge valuations they were doing last year.

Until January, life was groovy as an internet start-up. Over the past 18 months, hedge funds and other large investors eagerly swooped in and invest hordes of cash into consumer internet companies that showed growth in users — start-ups like Slide, Ning and Zillow. Many of these start-ups were making next to no real revenue at all. But the hedge funds were so flush with cash, they aggressively set the pricing of deals, bidding up the valuations of private companies — expecting revenues would come later. They effectively shut out venture capitalists from investing. For example, companies like Slide and Ning drew backers like T Rowe Price and Legg Mason, which injected tens of millions of dollars at valuations in the several hundreds of millions (our coverage of Slide and coverage of Ning, respectively), far outbidding more cost-conscious venture firms.

However, since January, interest from a key group of investors, the hedge funds, has evaporated — almost overnight. With the economy tottering and the credit crunch hitting home, hedge funds such as Artis (which set off the hedge fund binge in the first place, when it backed YouTube, showing hedge funds could make money from start-ups), Galleon and Integral, have shut their faucets. These firms have many of their assets in public stocks, and with stocks declining, they’re reluctant to look at anything that is more than a year way from having a clear shot at going public — meaning few Internet companies in Silicon Valley.

Case in point: Meebo, the site that lets people use a single sign-on to IM across different platforms. It’s got user traction, but its no where close to going public, and has minimal revenues. That company has been on the road trying to raise $20 million, we’ve been told, but it hasn’t drawn the inflated valuation offers of yesterday.

(By the way, we reported earlier about the rush by other start-ups to go out and raise cash before things got ugly; turns out, they were smart.)

Meantime, other large investors, such as T Rowe Price, AllianceBernstein and Legg Mason, which place huge amounts of money (some of the manage hundred of billions of dollars, if not trillions) in the initial public offerings, have also tightened their habits. They haven’t turned away from private companies, but they’re only backing companies close to going public, two years out or less. They backed Slide and Ning because those companies are run by seasoned entrepreneurs. Going forward, with markets the way they are, they won’t be backing any more Slides or Nings, as those companies are still very early. Allen & Co, the New York investment bank, initially acted as a broker for some of these bigger investment companies, hooking them up to Silicon Valley deals, but we’re being told it is having trouble drumming up interest these days.

That’s not to say that a company like Meebo is in trouble. Venture capitalists are still eager to back companies with traction, and some of them learned that downturns are the best time to invest in private companies — precisely because valuations come down, and they know they’ll get a better return when markets improve. Meebo raised money last year, and it is still seeing solid growth in its userbase, with some 29 million unique visitors last month across its networks. It will likely raise cash soon, but it probably won’t be at the stratospheric levels of 2007. Meebo’s Seth Sternberg declined to comment on his fund-raising plans.