Only a very, VERY few friends know. One, because he's someone I learned about electrostim play from at a talk; we started talking toys, and it just came up. The other... I think all of two friends who know, only know because to give them advice on where to start, I had to fess up to knowing a thing or two about the topic. They're both very dear friends whom I know would never, ever betray the trust I showed by telling them.
There are other friends I might feel comfortable talking about it to... if I didn't have to worry about them blabbing. I've been outed as a sex toy reviewer to a group of people I didn't know once before. I'm not letting that kind of thing happen again. (thank goodness, they were a very open, accepting, and discrete group of college students; they saw how uncomfortable I was, and let it just be a non-issue)
None of my boyfriend's friends know. They wouldn't get it, or would think he was gay, or decide that people needed to know... etc.
