This is another of those crazy topics with arguments not focussed on anything other than disagreeing with people.
If we look purely at number of people on a machine playing games online, PC has the most. There are squillions of people playing MMOs that the consoles don't have.
If we consider number of gamers playing the sorts of core games we think of regards consoles, it's very different. COD and Battlefield sold for smaller numbers on PC than either console, meaning if you're playing these games, PC offers a much smaller number of online gamers to go against. Which is pretty immaterial as long as there's always enough to fill your game, but clearly some people want to argue anyway.
If you start to include online PC gamers in the US and EU, including countries like germany (which is the biggest gaming market in EU) and is primary a PC gaming haven.
Are you sure about that? I can't find any numbers other than a £900 report, but a GI.biz summary says Germany is larger than the UK now, but that's for all revenue including mobile gaming. So the simplification of 'Germany is mostly PC gaming and so their revenue is mostly PC games' is on very shaky ground. It's all about slices of pie for each platform, which are numbers we don't have.
If we look at something like Steam versus Live Gold and PSN accounts, there's no obvious 'most online' answer, especially when those simple account numbers don't tell you how many hours are being player. If, for example, PC had 10x the number of people playing only, but limited to a few games or 30 minutes every few days, whereas Live has less players but gaming more often across more titles, than regarding a given game like COD, you'd find more gamers on the console.