I'm sure there will be much arguing about this. I feel that social media works as a kind of advertisement in that I get to know about a book coming out etc. But as a sales tool, it is not particularly effective. The best sales tool is still word of mouth from people I trust.

It is probably better to spend more of one's time writing than doing social media. Most well-known authors use social media, but they are on it infrequently. Too busy working on their novels, I presume.
Plus I'm sick of people who follow me on Twitter and then when you go to their account it's hourly Tweets about their book. That's definitely much more of a turnoff than a turn on.