This is absolutely a personal decision and will vary depending on who you are and who you are trying to reach. For instance, a band and a stock broker could equally benefit from Social Media, but would use different sites and methods to do so.
The first key point in choosing which sites to use is deciding exactly what you want to share and who you want to share it with.
Next, you should think with comprehensive profiles and the four main types of content you will be sharing: video, photo, status messages and blog posts. Ideally, you would incorporate each of these regularly into your Social Media presence and plan to have a presence on at least one site that specializes in each. Following this, a simple yet extremely effective strategy would be to have a Facebook, LinkedIn, Flickr, YouTube and Twitter account, as well as a blog, in your Social Media Campaign.
Lastly, you have to know how much time you can allot to your Social Media efforts.
It is vital to plan the size of your Social Media presence according to what you can comfortably maintain. It is exponentially better to have some key sites always up-to-date than many that are outdated.
Although it is the case that sometimes updating your network can fall to the bottom of the to-do list, in order to be successful at all, you have to keep your profiles up-to-date. Therefore, it is a great idea to make Social Media part of your daily routine, just like checking your emails. If you can only allot 15 minutes each day, realize that 40 different profiles to update is not for you.
So, with a little research and following the three simple points above, you can choose the correct sites that will give you the greatest chance of success.