Twiducate is a new web application that says it was designed by teachers. It provides a private social network where teachers and students can communicate, with messages not visible unless users register and sign-in.
I could see this being useful for classes of younger learners where teachers or schools are concerned about their web content being public. For older students, I think Edublogs or other tools I have listed in The Best Places Where Students Can Write Online would work just as well, if not better.
One feature I do like about the site is that images can be directly copied and pasted without using their url address or uploading them.