Your parents probably told you that sharing was simply the right thing to do. But on the Web today, inducing people to share links has become big business, all about driving traffic back to a site and increasing ad revenue. Young companies like AddThis (owned by Clearspring Technologies) and ShareThis are the giants of this particular corner of the Web, syndicating their catalog of sharing buttons — at no charge — to major Web sites, and developing ways to make money by selling data about who is sharing, and how much, back to Web publishers and their advertisers.
via nytimes.com
share and share alike