Even web giants like Facebook and Yahoo generally aren't dealing with big data, and the application of Google-style tools is inappropriate. Facebook and Yahoo run their own giant, in-house "clusters"—collections of powerful servers—for crunching data. The necessity of these clusters is one of the hallmarks of big data. After all, data isn't all that "big" if you could chew through it on your PC at home. The necessity of breaking problems into many small parts, and processing each on a large array of computers, characterizes classic big data problems like Google's need to compute the rank of every single web page on the planet. But it appears that for both Facebook and Yahoo, those same clusters are unnecessary for many of the tasks which they're handed.
via NoSQL databases