Facebook quite possibly was more responsible because they're more popular. But they also appear to be a actively fighting the misinformation that these groups spread and thrive on.

Whereas Parler is the opposite: they practically celebrate how little they care about undermining democracy with misinformation.

Actually FB let it fester and didn’t do anything about it until recently. Sure, Parler exists to provide this needed freedom of expression to extremists and does not represent what it is but in terms of impact, FB has had a lot more and did nothing about it

appearances can be deceiving