The place a social media 'campaign' fits into your web plan depends (of course!) on your business, and the purpose and function you want your website to serve.
Sometimes, it's a natural fit, and some are a little harder to find their place. It is a time requirement, so there should be solid goals and reasonable expectations set.
Let's look at it as components, and how & when they fit together:
1.Your Website
The Main hub of your online presence.
Your website should have everything that a perspective client/customer would need to know so they'll do business with you - or not.
2. Blog -
Sharing your expertise,
gaining a following who are interested in what you have to say
and developing relationships based on exchange of ideas.
It may take a while to get your engines greased on this one, but the 2 things to remember to make this work for you are consistency (2-3 times a week is considered good), and content that people will want to keep coming back to.
Another big advantage of a blog is that it helps your google search ratings, since google pays a lot more attention to sites that are updated often.
Here are the major social media sites, and how they define themselves.
They give you a big clue as to how to use them.
There are a few blogging 'platforms' – typekit, blogspot and wordpress, which is the most widely used particularly by professionals.

3. Facebook -
"Giving people the power to share and make the world more open and connected." from facebook.com
A social sharing site - connect and re-connect with friends, and more.

3A. Facebook Business Page –
A facebook business page is a different intention from your personal profile page. You want to connect with friends, the regular page is for that. Your business page is just that. Another way to showcase your business, auxiliary to your website and/or blog, it has potential to be what you want it to be.


4. Twitter -
Twitter is known as a micro-blogging platform. It's another way to have presence.
It's good for sharing information via links and gathering a following of people who appreciate that information. Some of these links could be to your website or blog, but sharing information that you've found in other sources is very good.


5. LinkedIn - linkedIn.com my LinkedIn page: Mintzwebdesignboston
LinkedIn is totally professionally oriented, in tone, visual presentation, and the ways people connect with each other.
As such, it is great to use if you're a professional wanted to connect with others like you, but it's not a place to sell products or be "social"


6. Pinterest – Communicate visually.

I love this. It's how I've been communicating, or trying to, as soon as I could hold a crayon.
This is a little more sophisticated, but not much. It's loads of fun, and the variety of 'pinners' are is a wide as you imagine.

This is an over-simplification, so take it as that:
You can spend a lot of time on social media, connecting all over the place until you start to
wonder where the time went.
Who are you wanting to attract and what would make the most sense as a way to connect with them?

Main rule of thumb - Be where your customers/clients are. Talk with them.