I think it's still majority-USA, but the proportion of European internet users who mainly "do internet" in English is growing. It's no longer just the UK and Ireland, but these days probably a majority of the Nordic countries' users, a large percentage of those in the Netherlands, a several-million-large minority of Germans, a bunch of younger Romanians, etc. On Europe-centric English-language forums, Brits+Irish+expats no longer constitute >90% of the users, but as a rough guess more like half, though it varies greatly by community.
I would say the primary reason they "do Internet" in English is because of the wealth of US-produced content. Like it or not, the US (specifically LA / NY) is the center of the world for mass-market media culture.
I think that was the initial impetus, and is what made English the lingua franca. But now that it is, it has a life of its own where people are using it even for communication entirely between Europeans. If you want to have a site dedicated to Northern European politics, where Dutch, Swedes, Germans, Estonians, and Poles all post, what language is it going to be in? English, of course, even if not a single American or British person ever reads or posts. Why are all the Danish government websites translated to English? Not so Americans can read them, but so people from other European countries can read them, along with recent immigrants (who are mainly Germans, Turks, and Arabs). The EU institutions are moving towards English as the main working language for the same reason, because it's the easiest way for a German and a Swede (say) to communicate; with the French being the main holdouts.
In the Nordic region in particular the huge dissimilarity between Scandinavian languages and Finnish is driving it imo. If the cultural bloc were only Norway/Denmark/Sweden, I think it would settle on a kind of "Scandinavian" (their written forms are mutually intelligible). But that would exclude Finns from the Nordic family, so the working language has to be English, whether it's in formal institutions like the Nordic Council, or informal ones like http://www.reddit.com/r/Nordiccountries. I think this is only going to accelerate, and Americans will gradually become a smaller proportion of English-language online posters than they used to be, as the younger generation of Europeans who are comfortable in English become more present in anglophone contexts. But it'll be a while before there's a real critical mass.
