Twitter, the popular micro-blogging website, took another step towards making money yesterday by amending its terms of use to allow advertisers to reach its 45 million regular visitors.

The company, founded two years ago, has exploded in popularity but has held back from introducing ways to monetise its internet traffic. Its founders have said they wanted to concentrate on growth and not alienate account holders.

Users of the service post short messages, limited to 140 characters, and can choose to follow the streams of updates from other users. The service has attracted increasing interest from businesses and marketing companies, keen to make use of its "real time" news value.

Co-founder and chief executive Biz Stone said there were no immediate plans to introduce advertising but the company wanted to "keep our options open".

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

Advertising revenue is the time-honored way for websites to generate revenue while remaining free for consumers. Earlier this year Twitter was the subject of fevered takeover speculation after the company turned down a $500 million acquisition offer from Facebook.

The service's explosive growth has made Twitter the social network of the moment: worldwide unique visitors to Twitter's site reached 44.5 million in June, up 15-fold year-over-year, according to comScore.

Some are sceptical 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.

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.

Ads could be delivered on users' home pages on the website, or alongside search terms. There was speculation that Twitter could even allow advertisers to place ads in the streams of tweets but such a move would be bound to result in a user backlash.

Twitter has said it will be introducing paid-for accounts for companies which want to have a brand presence on the service. Many firms, from airlines to restaurants to computer makers, already monitor the website to boost their customer relations.

Mr Stone said earlier this week that new products on the site might include an “analytics dashboard” to help companies monitor tweets about their business.

Twitter, based in San Francisco and backed by venture capital, stressed its new clause on advertising 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...."

Twitter's new terms of use came as its biggest rival Facebook launched a slimmed down version of its site for people with slow or poor internet connections.

Facebook, which has more than 250 million account holders around the world, said its Lite site will be faster and simpler because it offers fewer services than the main website.

The service, which many are comparing to Twitter's simple, pared-down style, is initially available only in Indian and the US.

The options on Facebook Lite are limited to letting users write on their wall, post photos and videos, view events and browse other people's profiles.

Last month Facebook bought FriendFeed, an up-and-coming web service, in a sign that it plans to extend its lead over rivals such as Twitter. FriendFeed allows users to see what their friends are posting on different social media sites in a single feed. The acquisition is likely to boost Facebook's move into instant, real-time updates.

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