Usability and getting involved in social media isn't just about 'where to place links'. You need to understand the following:
1. what users want when they come to your site
2. attributes of successful social organisations
Users aren't likely to visit your social media pages because it doesn't tie in with any of the tasks they want to complete at your web page. Placement of these links is irrelevant as a very small percentage of users will have these links as their end-goal, and even if they do visit your Facebook page or LinkedIn profile they aren't likely to take any actions that benefit the organisation.
A more important consideration is how to engage users in being social regarding your site and organisation. The success of social features comes from consumers and users distrusting corporations, but trusting each other and engaging in conversation.
The idea is that if a friend is promoting or sharing or re-tweeting a particular organisation, you're more likely to trust that organisation and become a consumer of their goods or services.
A great way to encourage this kind of conversation is to provide valuable content to users and provide 'share', 'like' or 'email' functionality both at the top and bottom of the respective content - not in the header or footer. Users want to express themselves and share value with their friends and family.
Valuable content is both the content users come to your site to find (about your organisation, pricing structure, services etc.) and also content that they discover on your site that is valuable to them. You can create this content by analysing the demographic that visits your site, and providing interesting and relevant information to that demographic through blog or news articles.
You could also showcase valuable content from your twitter feed (ensure it is valuable for your demographic and consumers, not just marketing for your organisation) in a mini feed on your site. This lets users see whether or not the content is valuable to them - and if it is they are more like to follow you on twitter & re-tweet or share your tweets (i.e. engage in conversation with their network).
Implementing these features are more likely to give your organisation value and benefits than links to your Facebook or twitter profile.