The complaint does not require a monopoly on either side. It formalizes the reason you don't like the FB login requirement.
It is abuse of market power on both sides of the FB/Oculus coin to enforce use of their ecosystem. The market power of FB is obvious, so even if they are using it only to drive a few more ppl to create FB accts by buying Oculus, it is an abuse. Doesn't require Oculus to have much power.
Considering FB & Zuckerberg's long history of abusing privacy, predating even Facebook back to Facemash (which scraped Harvard students' photos from an online directory and set students to rank for hotness), and then Zuck either saying "sorry" or attempting to rewrite history, it seems the House is on the right track here.