Usually, it's the FTC that prosecutes antitrust cases; the DOJ hasn't had a high-profile antitrust case since it prosecuted Microsoft in 2011. This could be read in different ways: perhaps the DOJ is as serious about these cases as it was about the Microsoft case. Or maybe it's trying to gain points with a public growing weary of the power of tech giants.
The DOJ is very serious about these cases but not for any altruistic pro-consumer reasoning. Anyone who has been keeping tabs on the political climate knows the exact reason that this issue has become the center of attention. Likewise it means that the tech giants know very well how to bury these investigations: Stop banning far/alt right commentators.
The tech giants are definitely a monopoly and are hurting consumers so I'm all for busting them up. But the DOJ is interested in this for the wrong reason and it's going to make it very easy for the tech giants to dangle some bait, that won't harm their marketing approaches, to kill off the investigations.