Looked it up on the Urban Dictionary, j/k is just kidding. js is just sayin, and j is just or even just sayin....It's for texting. I abbreviate all the time when I text cause of my charges.
This happens to me all the time. They probably aren't typing "j", they are typing and it turns into a "j" when they either send the e-mail or post the post. I don't know why this happens, but I see it on my e-mails and e-mails to me where my smiley face turned into a j. Sometimes I'll see it as a smiley face, but then later when the person responds to me and I see my post below theirs it has turned into a j. I'm sure some techie will know why this happens.