Yes, a lot of people use Facebook and Google but that doesn't mean that there aren't 50 bajillion means of communication. Network effects are strong but ultimately they don't prevent you from communicating with anybody. The person on the other end of the line who has a strong preference for their chosen form of communication does. There's also basically no barrier to switch, I mean I've got 12 different messaging apps on my phone right now and it's not even close to a burden. And that doesn't even include email and IRC. And when it comes to broadcast communication which this article seems to be focusing on Facebook and Google are by far not the only sources of broadcast. Hell you should be looking at the consolidation of print and broadcast television if you want to see a true cartel on information distribution.