Before the Social explosion, a website would mostly be a one way conversation (exluding forum and member sites of course) which would at best include a Contact Us form as the only way for a user to interact with the site and even then, only with the website admin.
While those days are now behind us, we now have a large number of ways for someone to interact with a website and ergo a brand or a company via a multitude of social media channels. Facebook, Twitter, Google+, Pinterest, Instagram are but a few and while it may only be a small icon or button for each they quickly add up and you end up with a website in Social overload.
Which social media platforms you add to a website really comes down to the website strategy and how you want your users to interact with your site (and you with them).
As a base you can’t go too wrong with Facebook, Twitter (and dare I say) Google+, which is the second biggest social network in the world. Then add other networks that your brand is able to support and that fit your brand’s strategy. Having Pinterest added to a IT support or services website, for example, may not be the best fit.
The second issue is then usability. These icons/buttons will need to be added in a way that is easy for the user to understand, but does not act as a detriment to the website’s core function, i.e. you dont want to loose the user. You will not be doing the user any favours if you have buttons all in one section that hides or covers content. The buttons also can’t be in a location that is hidden or cornered.
AddThis does this well, allowing non-technical users to be add a small piece of embed code to their website. It then allows for positioning and styling of social buttons. Of course this isn’t the most optimal built-in solution, but it’s a way to get Social integrated quickly into just about any website.
At the end of the day the most important thing is not to lose the user, but at the same time allow them to connect with your brand and be part of it.