There's actually a chat with direct message in PeakD. It's the button just to the left of the notification bell but if that member isn't using peakD they won't see it. You can also send a wallet message. It's kind of cheezy but people send 0.001 Hive along with a message. Those can go unnoticed as well and some actually see it as 'wallet spam' and frown upon it. I think one can send encrypted messages that way as well but I can't remember how. Sometimes people will also simply send a message under an old post and talk. People do that to me anyway so I assume they do it to others. I don't mind but some people freak out if you're 'off topic', which is weird but whatever floats their boat I guess.