The most common that we have seen is that the social media icons in an email signature link to the business account. This is because these accounts are branded for the company and are usually managed by a trusted person who understands that what they post is representing the company. This allows your clients & customers to follow the account and get updates on what is happening at the company.
Linking to employee's accounts may be more tricky because they are unfiltered and anything can be posted to these accounts. The employee may not always be thinking about representing their company with what they say online. That said, linking to professional accounts like a LinkedIn profile, a portfolio, or a professional blog can be valuable uses of employee's social media when used well.