Can the UK government regulate US-based social networking sites?

For some time now, the social networking phenomenon and issues around safety have been very much in the media spot light in my home country, the UK. And the past few weeks have been no different -- except it's the UK government that's been setting the media agenda as it grapples with how to regulate children's use of sites such as Bebo or MySpace, despite that fact that these companies reside outside of the country.