If it's a single provider doing it, without anything close to a dominant market share (the current situation), I agree. It would become more problematic if it became de facto the standard communication platform, though, or even a large share of it. For example, if Facebook messages replace email for 90% of the population, you will find it very difficult to avoid using Facebook messages.
I don't see it as much different than the State, really. Companies with smallish market share are like local governments: relatively harmless, because you can just avoid the ones you don't like. Bigger governments and companies get increasingly hard to avoid; it's easier to move out of Pittsburgh than to move out of the U.S.A., and it's easier to avoid Google Plus than to avoid the big-4 telecoms.
If a G+ or Facebook achieve 90% of communication volume, it means that anonymity isn't an important factor for most people and you could in fact argue that the high market share is due to the fact that there are less problems with spam and abuse because there is accountability. If a significant amount of people desire anonymous communication, there will be a communication platform for them.
Also you can avoid G+ and the big-4 telecoms much easier than you can avoid living in a state. The need to live is much greater that need to have internet access, no matter how you slice it.
You can avoid living in a particular state more easily than you can avoid internet access, I'd argue. In 2011, the hardship of going without internet is much higher than the hardship of moving from California to Nevada, for example.
I do agree that national-level laws are more coercive, in part because many countries are just very large, and in part because immigration laws make it hard to move.
I don't see it as much different than the State, really. Companies with smallish market share are like local governments: relatively harmless, because you can just avoid the ones you don't like. Bigger governments and companies get increasingly hard to avoid; it's easier to move out of Pittsburgh than to move out of the U.S.A., and it's easier to avoid Google Plus than to avoid the big-4 telecoms.