I’m not entirely sure the argument that the MegaUpload case and this Imgur case aren’t equivalent really holds up here. Imgur are collecting data from minors in the UK, and presumably selling that data (otherwise, why bother collecting it). Which is a crime in the UK (and the EU). You’re not allowed to collect people’s data and sell it without consent, and minors can’t provide consent (that’s basically what makes them minors).
Presumably Imgur sells adverts, and likely sells adverts to UK buyers. So for pretty much all the reasons you outline that the US should have jurisdiction over MegaUpload, the UK should have jurisdiction over Imgur.
I also take issue with the idea that being paid for something is the bar that we should be assessing a companies involvement in a country by. Obviously conducting monetary business transactions in a country is a very strong argument for jurisdiction. But it’s less obvious to me that breaking local laws, without making a profit, should somehow exempt a company from that countries laws. Particularly when the laws in question deal with how a company directly interacts with the citizens of that country.