I don't think there's been any noticeable impact. What you will find is that there are a number of online petition systems in the U.K. that the public will use. The 38 Degrees website has an e-petition system as well.
What we're tending to find is that members of Parliament individually may get fewer written petitions presented to them at the constituency level, because a lot of their constituents are using online forums. But members of Parliament might not have intended to present, or perhaps in the past actually presented, those petitions in Parliament anyway, as very little happens with them. Effectively they end up in the bag behind the Speaker's chair—quite literally—so they're not regarded as a particularly effective process.