Twitter opens door for advertising

Twitter, the two-year-old venture capital-backed company that lets people send an unlimited number of 140-character messages, is just now beginning to ramp up efforts to monetize, or gain revenue from, its popular site.

On Thursday, it revised its "terms of service" to specify that it may run ads.

"We leave the door open for advertising. We'd like to keep our options open, as we've said before," founder Biz Stone wrote on Twitter's official blog. blog.twitter.com/

Advertising revenue is the time-honored way for Web sites to generate revenue while remaining free for consumers.

Explosive growth in social networking is attracting interest: worldwide unique visitors to Twitter's site reached 44.5 million in June, up 15-fold year-over-year, according to comScore.