The policy become necessary when your employee have social interaction over internet with a lot of people all over the world and you don't want for this to interfere with your business. There are a lot of cases in the last few years about people losing their jobs because they have commented on Twitter or Facebook or they have posted on their personal blogs about their experience in your company or about their opinions about your clients.
Don't be very restrictive
You can not forbidd people to enjoy themselves and use social newtork. Also, don't creat a list with do not share things: it will only be longer and longer with time. It's better if use the policy to set some guidelines instead of do not do things.
Don't forget to specify how much time people should use social media
For some people in your company promoting your business online and offline is their jobs, they do it daily. But for the most employee social media interaction is just for fun. If you don't make it clear they can spend their time on social networks instead working for you, as they should.
Encourage the people you are woking with to use further social media, especially if they are well-connected (they have lots and lots of friends), but don't let them turn into a bad habit.
Make your social media policy very clear, but do not enter in details
I mean you should be focused about activities, connections, relationships. Social networks are perfect in promoting your products and services, but these will probably change in time. If you are clearly asking your employee to promote your business and its every part you will have to update the policy every time you are adding, updating and removing a product or service. On the other hand, this will irritate your employee and they will no longer enjoy your company.
Adapt and update your policy
Times are changing, we all know it and we notice it every day. Do not include in your social media policy old and outdated stuff (ideas, rules). If something changes in your business policy (a new market, a new section, something big in general) and this is relevant in your social media policy update it right away. Don't postpone things because problems might appear.
Do not create too many social media policies
Considering how many social networks exists over internet (some of them well known, with millions of users, others with only a few thounsands) and how many of them appear and dissapear every year, you can be tempted to create a policy for each them (one for Twitter, one for Facebook, for personal blogs and youtube). Don't. Use one general social media policy to consider all possible social media and customize it for your business.