Android: Google acquired the 22-month-old startup in 2005 for an estimated $50 million. In 2013, the mobile software platform powered 79 percent of new smartphones worldwide. Android co-founders Rich Miner and Andy Rubin are now execs at Google, working, respectively, in Google Ventures and on robotics projects.

Bebo: Long-suffering AOL paid $850 million for social networking site Bebo in 2008, hoping to do battle with Facebook. From a peak of 40 million users in 2008, the site was effectively closed down two years later. After an interim sale to an investment group, Bebo was repurchased by its co-founders in 2013--for $1 million. The "social bookmarking" site, with some 300,000 users, was acquired by Yahoo in 2005 for a rumored $10 million to $20 million. In 2010, Yahoo decided to sunset the service and focus more on its own Yahoo Bookmarks, then sold in 2011 for a rumored $5 million to two co-founders of YouTube.

Dodgeball: In 2005, Google bought this pre-Foursquare location-based social network for an undisclosed amount. Two years later, co-founder Dennis Crowley bailed from Google and started Foursquare. Google shut down Dodgeball in 2009.

Geocities: In 1999, Yahoo paid $3.6 billion in cash for this personal-website hosting platform. After nearly a decade watching user numbers plummet, Yahoo finally pulled the plug in 2009.