Hire professionals who are social media savvy. The gold in using social tools doesn’t come from one person in marketing or communications running your Facebook page and your Twitter account, it comes from the people within your company who are able to build on and leverage the knowledge and relationships in their own networks and in their own fields.
Make social media a desirable skill for people to have on their resume, not a position to be filled. One person is not going to get very far. Influence and the web are built on incremental transactions. Imagine how much splash your entire organization (or even a handful of them that are interested in doing so) can make in comparison to one or two people trying to keep things afloat.
If you are going to hire a social media professional, hire them with the goal of teaching and inspiring the rest of the organization on potential uses of social tools in their specific day-to-day operations. Otherwise, hire those who network well online, have a desire to continue doing so, and enable them to bring this skill into full play. With the experts in each department tending to their own knowledge base, imagine how much farther your company reach will be then.