At least kinda along these lines: Thunderclap, a thing that arranges for many people to send out social media messages simultaneously conditional on enough people declaring willingness to do so.

(Note 1: using this means giving the people who run it more access to your social media accounts than you may be comfortable with. Note 2: I haven't used it myself and have no first-hand knowledge of how well it works or how suitable it is for political activism.)