En español | So you found a high school heartthrob on Facebook and polished your resume on LinkedIn. Yet social networking isn't all "friending"; there's also the expertise you can gather from strangers—a phenomenon known as crowdsourcing.

Amazon pioneered crowdsourcing by posting readers' reviews of the books it sells. Now online ratings are ubiquitous—from Angie's List to Yelp—and Twitter users call on their followers to answer all manner of questions. But even if you don't have thousands of online pals to quiz, you can still call on the wisdom of crowds. Aardvark, from Google, makes crowdsourcing less obtrusive. You sign up, list topics you'd like to opine on (say, Chinese restaurants in Atlanta), then ask Aardvark a question. It scans your friends, friends of friends, and everyone in its database. Then it anonymously asks the 8 or 10 people most likely to have an answer.

When we asked Aardvark to find a specialist for a friend in Minnesota just diagnosed with prostate cancer, an answer came in a few hours from a respondent named Kerstin: "I can personally recommend two surgeons very highly," she wrote. Then she followed up with their names and numbers.