If you look at the activity in synchronous and asynchronous tasks the synchronous tasks win hands down! This might only be for this particular group as I have seen asynchronous discussions work well, but these are often tutor directed.
I expect that people will contribute much more to the Skype sessions than to the discussion board groups and this suggests that the discussion board is not an effective technology when used in this way.
There is too much expectation on the "students" to take control of their learning and this just does not work. Even the most active groups have only 50% of the group participating. Not having the discussion linked to assessment makes student apathy to the task even more likely.
While you can have lurkers in synchronous chats and this is not the end of the world, it becomes much more serious in asynchronous discussions as you end up with the sound of one hand clapping.