I really detest that term. The implication is that if you aren't out there doing something then you should just shut up and not be upset. To me that flies in the face of democracy and public discourse--words and opinions do matter.
If someone who is outraged but doesn't do anything is a 'slacktivist', then what is someone who is both not outraged and not doing anything about it? The implication of 'slacktivist' is that the second person is somehow better--that in order to avoid being a 'slacktivist' you should school yourself into being an always-detached philosopher of suffering, always reasonable, always balanced.