I am at the moment building up a web application made for people to buy and sell used book. The personn for which I am doing the website keeps talking to me about integrating social media features. I am myself a bit hesitant to adding such feature and I am wondering if adding any kind of integration of social media (such as Facebook) will be a something useful for the website or it will be just noise.
In my opinion, it is worthwhile to allow your customers to log into your site using their preferred method, be that Facebook, Twitter, Google, OpenID, or a site-specific account as a fallback. To this extent, it is definitely worth integrating with social media.
You may also want to consider setting up individual pages and the site itself as part of Facebook's "open graph" and embed a Like button. This is an incredibly easy way to allow your users not only to share your pages with their friends, but to have the fact that they liked it persisted in their user profile, and to have your pages show up in Facebook's search results. Huge (if somewhat questionable) marketing value. The only real downside to this is the extra load caused by Facebook's code on your page. Since it isn't critical functionality, you can always drop it if Facebook goes rogue :)
I think it depends. I wouldn't want to do tight integration where users can make comments on your site with their Facebook accounts or anything like that.
On the other hand, if you want to add something easy that could be helpful checkout http://www.addthis.com/ or http://sharethis.com/. They are easy to integrate tools that allow visitors to post specific links from your site to their social media accounts.