The biggest roadblock is the fact that B2B companies don't always have the right audience on many social media networks, especially if you're looking at it from a direct ROI point of view (sales). On Twitter, for example, a B2B can connect with its potential business customers, but those accounts are usually targeted towards their own consumers, controlled by a marketing department. These aren't usually the people who will be buying your services, so though you can make your name known, it's not as easy as attracting large numbers of people who buy your products directly.
There's no reason a B2B company can't be successful in social media, but it's a lot harder than going in as a B2C with established customers and a pre-made audience, essentially. You have to get more creative with your campaigns so people become interested in your business, regardless of whether they're customers or not, and that will work towards your goals particularly if the people you're targeting are employees of your sector. Branding and reputation, in my opinion, is probably the best focus for most B2Bs using social media at this stage.