It is unknown what impact the blogosphere has. There is some academic discussion of it acting as an echo chamber for ideas; basically that the messages of a brand, politician, political party or journalist become repeated and circulated across weblogs. Thus it can amplify messages or, by amplifying the messages of one party, it can reduce the impact of opponents. While there are active Labour and Liberal Democrat bloggers, it is the Conservatives that have the most organised, most followed and most sophisticated support in the blogosphere. Iain Dale and Guido Fawkes have become pseudo political celebrities as well as reasonably respected commentators (more the former than the latter) and there are a number of others such as Dizzy and Tory Bear with a significant readership. These may well be significant players among those who follow online debates. Guido mainly now seems to attack Labour at every opportunity, this critique of a party election broadcast is a particularly good swipe at the Labour spin machine
I have only seen one Labour blogger doing the same, Recess Monkey (whose amusing response to the video, if not to the criticisms made, is below) in fact left wing bloggers seem to also take a critical stance a lot of the time.
So the Conservatives have a lot of cheerleaders online while Labour, it seems, are failing to make any breakthrough in this respect. This may be symptomatic of the party's standing; it may be a failure to mobilise supportive bloggers, it may be that this simply not done in Labour circles. If there is an impact, and cheerleaders are important in amplifying messages and getting messages across to new audiences then Labour seem to be seriously lacking here.