Facebook has suffered its first drop in monthly users, according to numbers from web analytics outfit Nielsen Online.

Five per cent fewer people in the UK visited the site in January compared to the previous month. A total of 400,000 seem to have become bored with the social network and didn't bother to return.

A year earlier Facebook was still growing rapidly, despite the usual seasonal dip experienced online.

The slide is very bad news for Facebook. Rapid user growth is all it has to show for its massive investment in servers, and now it looks like even that story is evaporating. The only company bringing in significant revenue from Facebook is Rackable Systems, its hardware provider.

Advertisers - the people Facebook's venture capital backers hope will pay for it all in the end - have yet to swallow the line that Facebook represents a revolution in media or that targeting ads based on the personal information users give up is a useful new marketing technique.

Similar drops in interest have hit Facebook's competitors. Bebo has seen an eight per cent drop in UK users since October.

MySpace, meanwhile, has seen 14 per cent of UK users desert it in the last three months. Google, the undisputed motherbrain of flogging ad space online, says it has yet to find a way of turning the News Corp site's traffic into dollars.

Remember when Facebook was the future and supposedly worth $15bn? Stand back, because this fail is going to be epic. ®