As far as I know it does NOT affect SEO. It does increase the likelyhood of someone linking to you though. "IF" you want to know if this is worth it, look at your Google Analytics and see if people are converting on your website. Measure your FB and Twitter visits and assign a value to it.
My point here is not to focus on "will Likes and Tweets increase rankings". My point is "Do those Likes and Tweets turn into conversions".
Here is a secret of mine. Social Media does land me business, but not as much as Google Adwords. Adwords KILLS social media in my clients' industries. My clients only care about more money in the bank, not about FB Likes and Tweets.
LEARN how to use GA.