I would say you're just as likely to find NTs as NFs in the activism field. Indeed, I've been working my entire post-college life (ok so that's only 4 years) in social justice. For me the primary difference in impulse between the two archetypes are of driving intent:
An NT hears about a social injustice and thinks "well that's really dumb". An NF hears about the same injustice and thinks "that's really wrong". Neither one is more or less motivating/more or less worthy, as they both hit a core driver for each type. My first impulse, however, is less attached to direct service and more towards systemic problems. I tend to need little or no contact with those theoretically being helped by my actions, whereas I vitally need to see that programs are being put into place that will minimize/eliminate the injustice I'm reacting against.
Does that make any sense at all?
Yes.
I work towards the idea. Not the actual people involved. My INFJ gf on the other hand, is all about individual people. Yet, we still aim for the same resolution even if our motivation is focused differently.