However, I have a questions about voicethread: what is the purpose of using voicethread? It is clear that the main purpose of using voicethread is not to improve students' pronunciation accuracy. Since it is open to everybody, it is more important that more students post their threads and communicate through it, without caring too much about the accuracy. However, communicating on website is very different from communication in the real world. In many cases, we don't have much time to think what we say before we open our mouth and speak. Also, conversation floats in real life, so we cannot listen to the whole conversation again, even if we have problems understanding the topic.
Considering these things, I would say voicethread will work good with the purpose of reducing fears for making a speech and of practicing the basics of conversation (listen and answer) before students are exposed to real opportunities to use the target language. (It should be used more for speech fluency at first, and then speech accuracy at the next level.)