If you look to Reddit as an example of men, you'd think that the majority of the men on the internet were vile creepers who force themselves on women. If you look at any Dawkin's related forums, you'd think the majority of internet atheists were sexists racists scum bags that advocate the obliteration of anyone with a religious mindset. If you look at any radfem sites, you'd think the majority of internet feminist were rabid transphobe incapable of understanding that their concept of gender and sexuality doesn't exist cross-culturally.
But then you'll go into a defensive mode and exclaim "well Reddit doesn't represent all of us", "not all atheist are clueless", and you'd be right. So why is it that you look at tumblr and exclaim "that's the vast majority of social justice activists"? Especially when there's the Best of Social Justice thread showing their idea isn't as unfounded and inane as you preclude it to be.