Ubervu allows comparison of mentions of a pair of terms on social media sites. You can see the comparison of Gordon Brown and David Cameron here. I'm not sure what it tells anyone.
David and Gordon seem to be level pegging on Twitter, but all but invisible on Facebook in comparison, whereas discussions about Gordon dominate the pair on intensedebate which seems to be a YASNS (Yet Another Social Networking Site).
On the whole, I don't think that people often blog or twitter lines to the effect of "What a brilliant job David Cameron is making of presenting the Conservative Party's policies" (except by people who are paid to do so), so I would guess that on balance David has won the war of the social media sites so far.
