Apparently mostly facebook was used, not parler.
I will say that my preferred solution to the problem of Facebook (nationalization and open criminal investigations into its leadership) actually raises serious constitutional issues. The Parler case really really doesn’t - it doesn’t even raise good philosophical issues about free speech, just a bunch of dumb arguments.
Some of the specific, concrete planning of violence may have taken place on Facebook. That doesn't appear to have been taking place as much on Parler -- possibly because it was less private -- but there was plenty of generally violent rhetoric on Parler, just less specific. "We should kill this person" (Parler) versus "Let's all meet up with our guns here" (Facebook).
Husband: Two glasses of white wine in one night honey? You need to cut back for your health's sake
Wife: But dear, you drink a handle of vodka every night and have refused any attempts at intervention.
Husband: A-ha, classic whataboutism
edit: I'd love to respond to the people below me, but HN has decided that one post an hour is too much (despite having a 10 year old account and 4000+ karma)
I don't think anyone is suggesting that Facebook is free of guilt, but there is evidence that Facebook does at least try to keep their site clean, and for the most part they do. Its certainly possible that Facebook had (in absolute terms) more violence and planning than Parler, but as a percentage of the overall content, it's much smaller.
This doesn't excuse Facebook, they can do better, but it isn't hypocritical to say "the group with 10% violent content that tacitly encourages it isn't okay, while the group with .1% violent content that actively innovates in trying to keep things clean is okay".
Of course, that's only true if FB is actively innovating, and, well, that's questionable (Yann Lecun has made some concerning statements about Facebook's role, or lack thereof, in radicalization).