More likely these activists chose the most convenient platform for getting their message out. IG is popular and that makes it an obvious choice. Also keep in mind that activists will generally use what they are already familiar with, and Facebook runs three of the most popular platforms i.e. platforms that activists were probably already using before they became activists.

The only reason activists would switch to something else is if Facebook gives them a reason to do so. I doubt this one incident is enough of a reason.