There was a time in the early days of the Internet, when companies went on ill-fated missions to restrict their employees from surfing the web while at work. That effort worked about as well as Prohibition did in the 1920's. The Internet was new and exciting and nobody wanted to be told that they weren't allowed to check their Yahoo mail or read an online newspaper while at work.
The explosive growth of Twitter, Facebook, LinkedIn, Google +, etc. have created a "procrastinator's delight", as the number of diversions to getting any substantive work accomplished is never-ending. This conundrum is made more difficult by the fact that there are productive uses of Social Media for business, such as becoming recognized as an expert in a particular field, cultivating prospects, communicating with customers and creating unique, multi-format content for SEO purposes.
On the other hand, there is the purely "social" aspect of Social Media, which is completely separate from anything work related. I am not suggesting there is anything wrong with a "social" use of Social Media, but its impact on the work day can be significant from a productivity standpoint and should at least be acknowledged. I think it's important for companies to help their team members differentiate between time invested in Social Media that benefits the company, excessive business related participation (there are more important business tasks to be handled), as well as activities that are purely social.
Let me also note that people spending large chunks of time on Social Media, Fantasy Sports, reading newspapers, etc. is great for the online marketing industry, as most of those channels offer highly targeted contextual advertising opportunities on a cost per click or cost per impression basis.
It's easy to lose sight of how much time participating in social media can absorb. Try to focus energy on tactics that will move the company forward. This includes participating in channels (and groups within channels) where your customers (and potential customers) are interacting. Keep in mind that the SEO benefits of participating in social media are very genuine. Take the time to learn which activities will move the needle and which are less likely to make an impact. Lastly, while preventing personal use of social media at work is not a plausible stance for most businesses, frequent reminders on the importance of staying focused are valuable nonetheless.