I think the general theory is that if we allow them to continue getting bigger and swallowing up all potential competitors, we’re basically saying these four or five companies will run everything, forever.
Google is really the issue. They are dominant in search, browser, and maps, and they're half the mobile duopoly. It's very difficult to avoid using their products entirely and even if you did, their dominance allows them to dictate how the internet works, to a certain extent.
I couldn't care less about Facebook. Yet, while I certainly could live without Amazon (or Whole Foods), their service is very nice and WF has things I can't buy at other grocery stores around me.