@danja pointed out that there were RDF validation errors in SemanticTweet. Turns out that I hadn’t checked it closely enough before releasing. They key problem was that I was generating a single <foaf:knows> node, with multiple children, rather than one each.

A secondary problem is that some Twitter users don’t provide valid URIs in their profile description, so you have to validate them before creating a <foaf:homepage> node.

Anyway, all that is done, and a new revision of the service is now available.