Aside from Reputation Management (which is a huge reason why businesses get involved with social media) i think the main marketing objectives companies seek are A) additional avenues for promoting time-sensitive material; B) SEO/Link-baiting; C) Branding

For my clients, I have set up their social media channels as ways to find and attract new business prospects as well as potential new partners. Another way they use their social media presence, especially on sites like Linkedin, is to engage potential customers for their products and services so as to demonstrate expertise and knowledge. By going joining Linkedin groups that speak to the vertical markets they are targeting, they can look for people asking questions about solutions and services they offer and answer the question as part of opening up a dialogue.

Thank you all for your comments. I also believe that social media is shifting and the lines are blurring between personal and business, in which personal accounts are now serving a business purpose.