The JTB debate and the internalism/externalism dispute share a common assumption: knowledge is primarily an individual achievement. One subject, one belief, one justification. But almost everything you know, you know because someone told you. You know the Earth is 4.5 billion years old because you trust a chain of testimony extending back through teachers, textbooks, and geologists. You know what your city looked like a century ago because you read it. You have never verified the speed of light; you accept it on the authority of physicists.
Social epistemology, developed significantly by Alvin Goldman and others from the 1970s onward, asks: what are the epistemic properties of communities, institutions, and practices of testimony? When is it rational to believe someone on their say-so? What social arrangements are best for generating and distributing true beliefs?
Testimony raises the anti-reductionism/reductionism debate. Anti-reductionists (following Thomas Reid) argue that testimony is a basic source of justification, just as you are entitled to trust your senses in the absence of specific defeaters, you are entitled to trust speakers in the absence of reasons for suspicion. Reductionists (following Hume) argue that you can only be justified in trusting testimony if you have independent inductive evidence that this type of testimony is reliable. Anti-reductionism fits practice better (most children acquire knowledge before they can assess speakers' track records); reductionism seems more epistemically rigorous.
A further dimension is epistemic injustice, developed by Miranda Fricker in her 2007 book of that name. Fricker identifies testimonial injustice, the wrong done when a speaker's credibility is deflated because of their social identity (their race, gender, class). When a woman's expert testimony is taken less seriously than a man's on the same evidence, she suffers a harm that is simultaneously epistemic and ethical: she is wronged as a knower, not just as a person. This connects epistemology to political philosophy in ways classical JTB theory never anticipated.
Epistemology today is therefore a much wider discipline than the Platonic question of what separates knowledge from opinion. It encompasses the social structures through which knowledge is generated and withheld, the political dimensions of who gets to count as a knower, and the reliability of the collective practices, science, journalism, legal testimony, on which we all depend for most of what we believe.