The argument is that purchasing WhatsApp was anticompetitive and allowed Faceboook to cement their dominant market position. WhatsApp the app might be fine, but the competitive landscape is much weaker than it would have been if the FTC had blocked the acquisition.
I posted this elsewhere but I think Ben Thompson gives a very deep analysis of the issues in the competitive landscape here:
https://stratechery.com/2017/manifestos-and-monopolies/
The summary would be something like "any acquisition of a social network by another social network is necessarily anticompetitive and should be banned, or at least have the presumption of being illegal".
The only way that there can be a true competitor to Facebook is for a smaller social network like Instagram, or WhatsApp, or TikTok, to not get acquired and to grow until their userbase is bigger.
Zuckerberg understands this well; you can read his emails where he openly admits that buying Instagram is about preempting a competitor:
https://www.theverge.com/2020/7/29/21345723/facebook-instagr...
The money quote from Zuckerberg:
"There are network effects around social products and a finite number of different social mechanics to invent. Once someone wins at a specific mechanic, it’s difficult for others to supplant them without doing something different.”
“One way of looking at this is that what we’re really buying is time. Even if some new competitors springs up, buying Instagram, Path, Foursquare, etc now will give us a year or more to integrate their dynamics before anyone can get close to their scale again. Within that time, if we incorporate the social mechanics they were using, those new products won’t get much traction since we’ll already have their mechanics deployed at scale.”
I think you can apply the same argument to WhatsApp; by purchasing a rival social network, they prevent it from being a competitive threat.