The backbench business committee requires an e-petition to be taken up by a member of Parliament before it will allocate time for a debate.
I believe one or two e-petitions have hung around for a little bit before a member has been found to take them up, but I think I'm right in saying that all the petitions that have been referred to it have been debated, or at least there are a couple that have been scheduled for debate. The debate hasn't actually taken place yet.
In the current parliamentary session there have been three debates, with the two further debates scheduled. In the previous session there were five debates, and in the session before that, which is pretty much the start of this system, there were four debates. So it's about five on average a year.