From grassroots beginnings in 1970, Earth Day–which celebrates its 40th anniversary today–has blossomed into a global tradition.

Organizers expect more than a billion to honor Earth Day in 2010–but many will do so with Facebook rather than megaphones.

As part of the Billion Acts of Green, an initiative organized by the Washington, D.C.-based Earth Day Network’s Green Generation campaign, more than 30 million people will use social media to encourage green activities. One commenter on the Earth Day Network Web site named “Elroy,” for example, plans to “shower with a friend”–presumably to conserve water and electricity.