Facebook reached a milestone this week as its 200 millionth user joined the social networking site.

CEO Mark Zuckerberg noted the achievement in a blog post and launched an effort to put Facebook to use for greater good. A "heat map" on the site shows where the growth has occurred.

A new Facebook for Good page allows members to share stories about how Facebook has helped them aid and connect with others. Zuckerberg also said that the site has teamed up with 16 charities and advocacy groups to allow members to buy virtual gifts for each other through the launch of a new gift shop. The "gifts," or icons showing the charity, go on members' profile pages. Most of the money will go to the charities' causes, and Facebook won't keep any of it, but a small portion will help run the charities' administrative operations, Zuckerberg said.

"When we built Facebook in 2004, our goal was to create a richer, faster way for people to share information about what was happening around them," Zuckerberg said in the post. "We thought that giving people better tools to communicate would help them better understand the world, which would then give them even greater power to change the world."

Zuckerberg boasted that President Obama and French President Nicholas Sarkozy used Facebook to reach supporters, and several political groups have also used it to organize. "Creating channels between people who want to work together towards change has always been one of the ways that social movements push the world forward and make it better," he said.

Facebook rival MySpace has about 160 million members, according to figures from ComScore.

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