Paul Graham, founder of the influential, seven-year-old, early-stage investment company Y Combinator, tweeted out an interesting statistic today. “The total amount of funding raised by the 380 companies prior to the current YC batch is $1,048,274,000,” he wrote from his account.

That’s a lot of capital . But investors in Y Combinator companies are betting it’ll look like chump change once more of them mature. (Indeed, one person tweeted in response to Graham, “How about revenue?”)

A reminder of how Y Combinator works: twice yearly, entrepreneurial teams are chosen by Graham for what amounts to a three-month-long startup camp. The privilege of being chosen and the accompanying advantages it confers, including the ability to bounce ideas off some of the most power people in the tech startup industry, is enough for many thousands of people to apply to each session.

But Y Combinator teams – which agree to hand Y Combinator 6 to 7 percent of their company in exchange for what basically amounts to living expenses during those three months – have been so successful as a group that nearly all now go on to raise some amount of follow-on capital, and in some cases, a lot of it at extremely high valuations. (The “Demo Day” held after each session — when the entrepreneurs finally show the plans they’ve tweaked and polished over the preceding 13 weeks — is usually attended by every heavy hitting investor who can squeeze into the room.)

One of Y Combinator’s highest-profile success stories is expected to be five-year-old Dropbox, which was discovered and funded by Graham and that has gone on to raise $257 million, including from the likes of Accel Partners, Sequoia Capital, Greylock Partners, and Benchmark Capital. Dropbox’s most recent valuation was pegged at $4 billion.

Another expected home run is Airbnb, an online marketplace that invites users to find and rent any manner of accommodations around the globe, from couches to private villas. The company has so far raised $120 million, including from Andreessen Horowitz and General Catalyst Partners. During its last round, closed last spring, AIrbnb’s valuation was pegged at roughly $1 billion.

Others of the 380 graduates of the program include the travel search site Hipmunk, which has raised $20 million since its August 2010 founding; the blogging company Posterous, which was launched in the spring of 2008, raised $10 million and sold to Twitter earlier this year; and Scribd, the social reading service. Since its March 2007 founding, Scribd has raised nearly $26 million from investors that include Redpoint and Charles River Ventures.

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