I'm pleased to announce that on October 28, 2014, History of Science and Mathematics Stack Exchange1 entered private beta. On November 10, 2014, it entered public beta. As of this posting, it is 226 days old and going strong.
Hi, I'm HDE 226868, one of the pro tempore moderators on HSM. I'm posting this "answer" here as something of a thank-you to Math Overflow. We've had two questions migrated from MO to HSM:
Both of them went over quite well; the latter is one of the highest-voted and highest-viewed questions on HSM. Our thanks go out to Math Overflow for the migrations. They certainly helped HSM by drawing attention to it via those questions. We're a small site in comparison to MO, and much younger, so any attention that a popular question brings is quite welcome indeed. Thank you.
This answer also serves as a status update. We're doing pretty well, given that the topic at hand is something of a niche category. Our top tag is mathematics, used on 174 questions (out of 482); more questions may fall into the domain of mathematics but do not use the tag. Our Number 2 user in terms of reputation (and most active user), Alexandre Eremenko, is originally from MO, and there are many others that are active on both sites. I encourage more to come and check the site out.
But overall, thank you, Math Overflow, for the questions migrated, and for the users who sprang to help HSM.
1 We managed to get the title changed from "Math" to "Mathematics" soon after we entered beta.