Since the 1970s saying that you are an 'activist' has became increasingly common, as my totally lightweight linguistic research from Google Ngrams shows below:
Activist vs socialist
Activist vs agitator
Activist vs campaigner
It was the environmental movement in the 80s and 90s that helped spread 'activist' as a description. Swampy was the model here, and a very effective one:
Interestingly, the BBC drama about the rise of New Labour - The Project - gave some of the characters the rather implausible backstory of having been environmental activists:
I think it was supposed to heighten the narrative decline from youthful idealism to middle aged sell out, but just makes you dislike them right from the start instead.
Just as the students of May '68 were once accused of being 'a party in search of a proletariat' so you could ask the activists: who they are for? Squatting, a movement which once tried to organise homeless people to take over large numbers of houses, is now about temporarily occupying buildings with connections to well known bad guys or institutions in order to have a social centre or an alternative space. That's not a bad thing at all, but it does seem like a reduction in ambition. It also raises the suspicion that some of this activity is primarily cathartic or reproductive (in the sense of maintaining current levels of activists). I wasn't offended that the Boat Race was disrupted this year, but then I'm not offended by the Boat Race to begin with. Trenton Oldfield could at least have made the effort to say he wanted the return of EMA rather than just being against 'elitism'.
I mean throwing yourself at Oxbridge rowers isn't that far way from wiggling your arse at Michael Jackson is it?