Twitter, the fast-growing microblogging site now seeking ways to make money, expanded its terms for users on Thursday to allow advertisers to reach the Internet site's more than 45 million monthly visitors.

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. http://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.

Some analysts are skeptical that advertising will catch on in a meaningful way on social networks, arguing that companies are reluctant to juxtapose their brands with unpredictable, and potentially offensive, user-generated content.

Stone himself has said the company was wary of annoying its growing base of users by pummeling them with ads.

But other analysts point out that users of social networking websites tend to spend a lot of time on those sites, providing an attractive platform for advertisers to promote their brands -- especially if preferences are tracked.

Twitter kept its new clause on advertising open-ended, and stressed it was subject to change.

The services may include advertisements, which may be targeted to the content or information on the services, queries made through the services, or other information, the terms read. The types and extent of advertising by Twitter on the services are subject to change.

In consideration for Twitter granting you access to and use of the services, you agree that Twitter and its third-party providers and partners may place such advertising on the services....

(Reporting by Edwin Chan; Editing by Gary Hill)

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