Can any one tell me How Facebook application plays a big part on your daily deal site?

As we all know FB is that kind of site where number of traffic is coming in a day and from there traffic can convert eaisly on site. So these why it's going most important.

it play big role in marketing side

FB application helps to get the more traffic as well as brand visiblity in the Market. Its always benefitial for SMO process.

I would add that people started not to make so many searches for purchase on major engines as they receive from friends places from where to buy things.
So FB is not important only for traffic, but also for sales.