Mark Zuckerberg says people use Facebook "to stay updated on what's happening around them and share with the people in their lives."

Turns out it's true.

According to AddToAny, a company that provides Web publishers tools to let their users share content, more people use Facebook to share links than any other service -- including, to our surprise, email.

But watch out, Facebook. As a means of sharing content, Twitter is already about half as popular with only about one-tenth as many users.

This matters because content-sharers are the human crawlers that power both Facebook and Twitter's real-time search engines--which could turn out to be the way both startups end up making big money.

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