I think that so many times companies embark on a social media campaign either because their competition is doing it or they feel they have to in order to keep up with the times.
I agree that before you embark on a social media campaign you need to decide - What exactly do you want to achieve? This will determine exactly which platform is the best for you - be it Facebook Fan Pages or a Twitter profile.
Social Media is about building relationships and engaging in conversation. If a company is not gonna put the time into their Facebook page for example there is really not point in having one. Post things that you know your customers want to read about, engage them by asking questions.
If used right social media can help companies not only engage with their customers but also help them collect valuable market research that they would not normally be exposed to.
So in short - Don't just do it for the sake of doing it! Have a clear plan and know what you want to gain from your social media project :-)
~ emma.n