Not really. I've encountered a few redditors who are obviously very knowledgeable and likely are (or will become) fine historians, but I only know them by username.

Also, I'd caution against inflating this subreddit into something it's not. This is a great place for informal discussion and attracts high quality answers for what it is, but this is far from an academic and professional agora. Questions here are overwhelming extractive, meaning they're asking for ideas and information already researched from people who keep up with those topics. They rarely ask about developing topics. Also, answers here have an informality that one would never see in the professional world. There are no lengthy collections of footnotes here, no full explanations of historiography. Reddit answers are too small and too low-stakes for that and frankly, when I do that level of research, it's to advance my actual career and not for karma. If my content here became part of my overall professional work and image, I'd conduct myself a lot differently.

Also, the (usually simplified) explanations of things already researched that appear here do not lend themselves very much to new research or collaborations. For collaborations on new research topics and questions to take place we'd need to be discussing sources and ongoing projects pretty heavily, and I've just never seen it.

Further, when it comes to publications historians rarely collaborate (as in, an article published by 2 or more authors) unless the scholars involved are well known and well established (few of whom would probably venture to this subreddit anyway). Though I disagree with the lack of collaboration, it's still a reality at the moment.

Obviously, I can only speak from my experience. Maybe others here have developed these relationships privately, and will share their perspectives here.

Has it led me to actual collaborations and publications? No. Has it led me to think through my work a little differently, and in that way affected my publications? Yes. The effect is subtle, but it's there. I was finishing my book manuscript when I started here, and until then I'd been pretty isolated from people beyond my own department and institution (and a few other colleagues around the country). This was a helpful move.

That said, I've met other academics through here, so I've made professional contacts of substance. This isn't the first place I've been where I've met other historians of Africa or imperialism as well, so I've got those professional contacts too. What /u/Borimi says is quite true; collaborations are not the norm even among people who meet in person, with the possible exception of conference proceedings and other edited volumes. I've had a chance to be involved with conferences and panels as a result of these contacts, so there's been an effect, but it hasn't been direct.

I can say, tenitively, yes. I am not a proper historian in the academic credentials sense, merely a lay enthusiast who's read too much in a small area. So I certainly never expected to collaborate with any historians, period.

Yet unexpectedly, I was recently handed an opportunity to complete a historical research project for work as part of a pilot project (I can't/won't go into detail because of confidentiality reasons). I started some research and quickly realized I knew two flaired users here who would be able to help me narrow my research or figure out how to define the scope of the project. Both of them have most kindly helped me to the best of their abilities, but, as the project is not yet complete, it's a bit early to really speak of collaboration or help.