are the actors using blogs as the medium to get near to the fans or just an easy way of publicity?

It is a way of publicity. But it does provide a better medium for them to express themselves without "twists " by media and also a direct way to communicate with their fans and followers. For instance, Amitabh's blog was created as publicity gimmick for a big blogging site. Certainly Amitabh as no intention to go for blog but now does have one.

But these blogs have big impact and no one can deny it.

It's only publicity.