Scratch

Actually, the Forums are really intended for things you want to say to all Scratch users at once.  For less public communications, please leave comments on each other's projects or galleries.  Communication by Comments, in addition to being more private, also has the advantage that your friends get a notification when a comment has been made.  So it's important to put at least one project or gallery on the website just so people can leave you messages 

There are many advantages of not having  a private system. One of them is that adults are less likely to contact children in an inappropriate way if things are always public and there is a record of it.