Jenny Levine makes a great case on The Shifted Librarian for adding a feature to your sites that allows users to easily share your content via social networks. Not only will this offer a way for your users to help spread the work about services and activities that you offer, but it can help them keep track of information they find so that they can revisit it later. This will be a benefit for users of mobile browsers, which are rapidly becoming a more significant portion of visitors to our web sites.
You may notice that I have followed her advice and have added the Add to Any service to this blog, visible at the bottom of every post (and checked to ensure it validates against XHTML and CSS standards, of course; a requirement I have for any add-on).