No it's not. However, they didn't send a cease-and-desist letter to the users that installed the plugin. They sent the letter to the researchers that built the plugin that uses the service Facebook offers in a way that is against Facebook's wishes. Therefore, technically, the letter is going to the correct party and for the right reasons.
Mind, I'm saying 'technically'. Ethically, I think Facebook (and other big social tech) should be researched more. In legal ways, this is how they should handle this.
I use vimwiki to keep notes. It's not synced out of the box, but as it's just a directory with text files, you could make it so. If I'm not with my laptop, I just make a note in Google Keep.
Mind, I'm saying 'technically'. Ethically, I think Facebook (and other big social tech) should be researched more. In legal ways, this is how they should handle this.