There is something pathological about this response by people who have taken the poke. Not everyone I know that got the poke reacts that way. There is a certain segment that does though. They are very vehement. I think its caused by cognitive dissonance.
You’re absolutely right. I don’t know of anyone personally that act like the pro-vaxers on FR. Even the incidences I know of where people tried to encourage someone to get it wasn’t in a bullying way like the pro-vaxers on FR. As you said, they put too much pathological and vehement effort into it. Makes me think there are a lot of paid trolls.