What is the best social media platform to drive traffic on my blog?

Both Facebook and Twitter can drive traffic on your blog. But if you are posting visual contents then using Tumblr, Instagram or Pinterest might be more effective.

its easy to go viral tru twitter..but both facebook and twitter are effective

Linked in, Google+, FB page, Twitter, 

The most important thing is knowing where your audience hangs out. You will need to do some intensive research to find out that. But it  is not difficult, it only takes some time.

You can even search for report on social media behaviour of people which might give you some insight into where your target audience are most probably hanging out. You can then fine tune your research based on it.

I believe that facebook comes in first place because of the number and the variety of the active users.

It depends on what type of blog you have, sure you can create accounts on all the popular Social Media platforms but more than often, not all platforms are equal and must at times co-relate to the content that is being promoted, i.e. if you are into photography then its Instagram for you, if you have an information website then twitter and Facebook. etc.

The first thing is to identify who your target audiences are. Once you've known and identified them, then you can carry on to find out where they are very active when it comes to social. With this in place, you can then create content that resonates to your audiences and be able to drive targeted traffic to your website/blog. There is no need to have traffic that won't convert to your targeted goals and KPI's.