Most definitely not can it replace SEO, teamed with SEO they can work very well together, the social end of the market is far different to the end users who are buying on Google
You engage differently with your social media than you do with your website, I have found with the A/B split testing on adverts including the testing with adverts vs page likes - the page likes goes well and you grow your audience and page but with the selling of products - well they don't convert so well
They land on the website, complete the shopping cart but don't check out
I am always rolling out new ads and testing, it could be just me and I am selling the wrong message or image - I certainly hope it's not that!
I do believe you need a bit of both these days, you can network much better with social media, reach people you never thought possible to chat with and start new friendships with some degree of ease, I do not think that social media will ever replace a good SEO'd website but teamed together they are a strong force - a good balance.
Many Facebookers for example are there to play games, chat to friends, post images on people's walls, create events and share their own images (a good percentage of users I would say).
I used to run support groups on Yahoo!, they missed a thing or two where forums are concerned - missed the boat - Facebook came along and today I am still trying to delete that Yahoo! group because Facebook grew so quickly it was impossible to manage them both successfully - Yahoo! back in the day had the best forums of that manner, now they pale in comparison and are a pain to use and manage.
So in my long winded way, no I don't think it is an alternative to SEO