I adore PZ Myers, but I think he's tilting at a windmill. Yes, dictionary atheism is boring and reductive, but just being boring and reductive doesn't mean it's not a thing. I think it takes a degree of stupidity to be both an atheist and against social justice, but stupidity is sadly common. (If you complain in the comments that you are an atheist, not for social justice, and not stupid, I will simply reply that you're mistaken about one of those assertions, probably the last.)
Social justice is about... well... social justice. You're either for social justice, in which case you're a good person, or trying to be one, or you're not for social justice, in which case you're a bad person. To be honest, I really don't care whether you're an atheist, a Christian, a Muslim, a Satanist, a Zoroastrian: you're either for social justice or against it. If you're for social justice, you're on my side; if you're not for it, you're not on my side. (This is a moral issue, not an analytic issue. I'm sure you think you're a good person — everyone thinks they are — but I really don't care what you think about yourself.)
Trying to pummel the "atheist community" into focusing on social justice is a mistake, precisely because the atheist polemic against religion has largely succeeded. It no longer takes unusual perspicacity or courage to become an atheist: any old doofus can be an atheist, and many of them are. And even the religious people — except for a few nutjobs that most everyone, even the doofuses, knows are nutjobs — are defensive about their religion.
I don't believe any gods exist, but I'm no longer an "atheist" in the political sense. I'm not a member of the atheist community, but that's because atheism has become so ubiquitous that an atheist community makes about as much sense as a community of people under six feet tall: we have nothing in common but a triviality.
Instead, I just straight up self-identify as a member of the social justice community. If you want to be for social justice, I'm not going to ask about your religion or lack thereof, any more than I'm going to ask about your sexual orientation or taste in literature.