The dorm mates who pranked and coded their way into the social lives of Harvard students may have grown up and gone their separate ways, but Facebook the company--still helmed by founder Mark Zuckerberg  -- has graduated into an international leviathan of stunning power. More than 400 million souls (as of this February) share their lives to an exhausting degree: They upload 2 billion photos, 14 million videos, and 8 billion bits of other information a month, and drag their identities and networks with them to 15,000 Web sites a month through Facebook Connect. As a result, Facebook has become the platform of choice for major brands, political candidates , scrapbooking moms, and social causes looking to "engage" and "converse" rather than merely "sell" and "broadcast." In September, Facebook became cash-flow positive for the first time. Top 50: No. 1 
The Internet's most popular search engine spent 2009 crawling for world domination: After releasing the Nexus One  (aka the Google Phone) and snaring mobile-ad firm AdMob , Google announced that it had doubled its mobile audience in 2009, to 25 million searchers; in total, it commands 86% of the mobile-search market. Meanwhile, YouTube began streaming authorized TV clips, shows, and movies from ABC, BBC, MGM, and others, and splitting ad dollars with their owners. Then, there was Google Wave , a marriage of social networking, writing, and photo sharing designed to replace email. And oh, yeah, overall revenue was up 8%, to $23.6 billion. Top 50: No. 2 
The 70-employee company, already the de facto standard for measuring Web audiences, sifts through its mountain of data to determine who's clicking on ads, visiting sites, and buying products online. Then, it markets users that have the same characteristics to media buyers--9 of the top 10 in the U.S. are customers--as well as CBS, ABC, and MTV Networks. The company plans to double in size this year. Top 50: No. 46 
Year-over-year traffic has grown more than 400%; virtually every major company, from Comcast to Wachovia, has opened an account; valuation is roughly $1 billion. Yet it remains to be seen whether Twitter  can win back the buzz--and the traffic--from popular third-party apps, such as TweetDeck and Echofon. In October, Twitter got $25 million for inking deals with Microsoft (Bing) and Google to add live tweets to search results-enough, reportedly, to make the company profitable. Top 50: No. 50 
The news-discovery site, in danger of being eclipsed by Facebook and Twitter, and no longer the target of acquisition buzz, suddenly found something that had eluded many in the social space: a path to profitability. Some 40 million users still Digg  nearly 20,000 submissions a day, but now they're also clicking on ads. "We're making money," says a bemused Jay Adelson , Digg's cofounder. Its experimental new ad system, in which ads are voted up or down and unpopular ads are dinged with higher costs, has stoked the interest of its traditionally beer-soaked boy base, and inspired marketers to offer better, more entertaining ads. "Digg has stumbled onto the future of online advertising," says Silicon Valley luminary Marc Andreesen . Expect a massive site redesign in early 2010. Digg executives also coach media sites on their own paths to profitability. Hurry up, boys.
Part sales management tool, part developer platform, and part mashup of public and private customer-insight data, salesforce.com is the undisputed leader in helping sales hordes track prospects and win business. New in 2009: Service Cloud, a customer-service software system that also analyzes comments and complaints on Twitter or Facebook; a collaboration with Cisco that offers an online-call-center capability; and Chatter, an internal Twitter-like feed service that will let sales forces interact, securely and in real time. Last November, the company announced third-quarter revenue of $331 million, up 20% year over year.
7. Wolfram Alpha
Physicist Stephen Wolfram set out to reinvent the search engine. Instead of giving people a shopping list of random Web entries, Wolfram Alpha  presents expert answers from databases around the Web, and it is capable of complex visual modeling, such as comparing stock and economics data. Its debut in 2009 rattled Google, which rushed to get out its own version of a visual data-modeling tool. Last November, the company announced that Microsoft's Bing will feature W/A searches and data .
On You Tube, the most viral videos showcase quick, one-off events, from a baby biting a finger to a man doing the Macarena: What's missing, says John Ham, co-founder and CEO of video-broadcasting site Ustream, is "a feeling of immediacy." No more. Since launching a pay-per-view service and its Social Stream feature, which allows users to post video comments as status updates on Twitter, Facebook, MySpace, and AIM, Ustream has become the go-to place to broadcast live events --say, the Twilight: New Moon premiere, Michael Jackson's memorial service--and grown its audience exponentially. In January, the site attracted 70 million unique viewers, up 500% from the year before.
As the real-word housing market collapsed, Zillow--an online network that connects and informs home owners, buyers, sellers, renters, and assorted real-estate professionals--soared to record heights: Monthly traffic averaged 8.2 million unique visitors in 2009, up 57% from the previous year. Among the features driving it are a proprietary "Zestimate" algorithm that appraises property values, an iPhone app that overlays housing prices onto Google Maps, and a revamped listings page that includes rentals. Additionally, more than 170 newspapers, including The Tampa Tribune and Seattle Post-Intelligencer, began using Zillow to power their real-estate Web sites.
As a startup, widget maker Slide became famous for superpokes and throwing sheep on the Facebook platform. But in 2009, the company switched strategies, abandoning a Web-advertising model to become a hybrid new-media studio that works with the likes of AT&T, Ashton Kutcher's Katalyst , Nestlé, Publicis, and Starbucks to create commercials dolled up as entertainment. Through its branded Funspace, the company now serves some 10 million videos a month and is the largest video-distribution platform on Facebook. Founder Max Levchin cautiously predicts profitability this year.