People use Facebook/Whatsapp/Twitter/Reddit for many of the same things they used to use email and personal blogs to do in 2005. But with centralized control and the ability to shape what messages get broadcast to whom, it's far too great of a concentration of power.
Google is probably the most extreme. Consider the "Evil Larry" thought experiment. If Larry Page woke up one day in 2016 while he was still CEO and decided to use Google and YouTube purely to further his own goals, how many national elections could he tilt while maintaining plausible deniability from the outside?
My guess is, it would be quite a few. It wouldn't have to look that different. Counter-narrative results would still show up in search. They'd just be down-regulated. Links and videos supporting EL's preferred candidates and positions would be up-regulated and be shown up perhaps 10% more often than previously. With plausible deniability, election after election could be tipped by a few percentage points. That's often enough to change the outcome.
There are almost certainly some internal company safeguards that would have made this kind of crime a difficult, uncertain proposition and I do NOT think the real Larry Page would ever have done this. But being completely unconcerned would be a grave mistake. Giant tech companies already lobby extensively in nations all over the world and do not have goals perfectly aligned with each of those nations—e.g., Free Basics in India.
It's naive to dismiss concerns about such extreme concentrations of power being wielded to suppress speech along political lines simply because we're currently benefactors of the action.