2. Twitter - Five mobile social networks you need to follow

Four years after it first launched, Twitter continues to grow--in mid-April, co-founders Evan Williams and Biz Stone announced the microblogging service now adds 300,000 new users a day and generates more than 65 million tweets daily, translating to about 750 tweets sent each second. Usage scaled new peaks this summer, reaching a record-breaking 3,085 tweets per second after the Los Angeles Lakers wrapped up the 2010 NBA Championship on June 17. Central to Twitter's growth is its increasing popularity among mobile subscribers: According to a recent study issued by research firm Compete, 33 percent of smartphone Twitter users primarily send tweets via their handsets, and 33 percent of these consumers prefer to read tweets on their phone. In interviews, Williams has touted the mobile platform's critical importance to Twitter's future, stating "We believe we can enhance people's lives on the move" and adding that tweets are confined to 140 characters expressly to fit into the body of one text message.

No less important, Twitter also continues to evolve, making a number of acquisitions in an effort to expand and improve its platform. In April, Twitter scooped up Atebits, developer of the Tweetie client for iPhone (since renamed Twitter for iPhone) as well as messaging infrastructure provider Cloudhopper--two months later, it purchased Smallthought Systems, developer of the Trendly analytics service. The deals have nevertheless fueled concern among Twitter developer partners who fear the company will now build more applications in-house or purchase services that bolster its overall vision, in the process making third-party apps obsolete. Williams did little to assuage those concerns, telling The New York Times Twitter will continue to build apps and features it needs, even if third-party developers already provide them, and added that Twitter bought search solutions provider Summize in 2008 because it realized it needed a search engine. "There could be other stuff like that, that completes the platform and makes it better," Williams said. "Since we're still evolving, that may happen more."

Twitter is also making money, introducing Promoted Tweets in mid-April. The initiative features ads that show up when users search for keywords tied to advertiser campaigns--from there, Twitter will eventually insert user-relevant promotional posts into the Twitter stream based on metrics like geographic location or shared interests among followers. Brands including Starbucks, Best Buy, Virgin America and Bravo have already signed on. A new digital commerce service, Earlybird Exclusive Offers, followed in early July, promising time-sensitive deals to users who follow the @earlybird tweet stream. According to Twitter, @earlybird deals are created expressly for the Twitter community; advertiser partners will determine the terms of each promotion, including availability, amount and price, with Twitter earning a cut of resulting revenues. The concept parallels social commerce services like Groupon and Woot, which also offer limited-time deals to registered consumers--Twitter suggests future evolutions of @earlybird could include location-sensitive offers as well as bargains tied to specific merchandise categories, like fashion or music.

Twitter's ongoing transformation poses one major question: Is it still a social network? Co-founder Stone argues otherwise. "That's been a myth since the beginning," he said during an appearance at June's World Innovation Forum. "We're much more like an information network or a source of news." Regardless of how you define it, one thing's clear: Twitter is redefining how the world communicates.