Its critics complain about the inanity of its content and its ability to suck time and attention from already busy lives, 140 characters at a time.
But such criticisms are minor compared with the biggest question facing the microblogging service Twitter: can it make money?
Biz Stone, Twitter's Californian co-founder, acknowledges the challenge but says his company is confident it has found a solution.
Stone says the best strategy is to build a real-time search service, from which a revenue model will follow.
In the early days Twitter had a casual tone, inviting users to share their lives with the world. The slogan was: ''What are you doing?''
''When we started out we thought we just had this fun service,'' Stone says. ''And that's why the early critics said of Twitter: 'It's not useful.'''
Stone and his co-creators initially saw Twitter as a way of staying in touch with family or friends. But the ability to follow other people without an existing connection set the network apart.
People started to use Twitter in new ways: to organise protests in Iran after a disputed election; to tweet (including photos) about a bridge that had snapped a cable in San Francisco; and to report - before other media - that an airliner had crashed into New York's Hudson River.
''When we saw it used during emergencies or political insurrection or even used to enliven live events, we knew it was something bigger; something truly useful,'' Stone says.
He now sees Twitter as a way of sharing information about current events. ''There's this real-time element to it and the information is very short and quick. If you want to find out what's happening right now you turn to Twitter. Or … say you've just witnessed an accident - you blurt it out in Twitter.
''We have millions of people around the world reporting on what's going on. It's a very powerful discovery and search engine for what is happening at any given time.'' Hence Twitter's latest slogan, adopted in November: ''What's happening?''
Being fun - even useful - is not a business model, however. Twitter can have millions of users - a recent estimate was 75 million people - but without revenue such size becomes a drain on resources, even a threat to survival.
Stone repeatedly mentions making Twitter ''more relevant and adding value''. The plan is to offer users a way to tap Twitter for relevant information. ''What we need to do is find a way to get that information out of Twitter and into the palm of your hand in a timely fashion,'' Stone says.
He has considerable breathing space. Google and Bing, Microsoft's search service, have reportedly paid $US25 million ($28 million) for real-time access to Twitter, making the company profitable for the first time in its four-year life.
The Google and Bing deals point to the growing importance to the internet of real-time search.
But Stone stresses that he and co-founder Evan Williams have a ''bigger mission''. They believe the open exchange of information can have a positive impact on the world.
''On top of that,'' says Stone, ''we think that these kinds of systems can be very profitable.''
Twitter's critics will believe that when they see it.
