Um, neither WhatsApp nor Instagram were competitors to Facebook. Both of the acquired companies simply did something that Facebook tried to do, but very successfully.

The government can only prevent acquisitions and mergers, when such would greatly harm the consumer by giving a company too much control of a vital market, like health insurance or phone service. Moreover, it didn't stop banks from consolidating 30 separate entities into 5 between 1990 and 2005 or so.