A company's Facebook page can give you information on company news and job openings and provide insight into the company's mission and organizational culture.
LinkedIn is a professional network where you can "connect" to other professionals and companies in your career area. You can highlight your education, experience, and skills while researching companies and networking. Associations in LinkedIn are purely professional, so be sure to always present yourself in a professional manner.
Twitter allows you to "follow" other industry experts and professionals to hear industry updates and useful information. Many companies recruit by announcing job vacancies, sharing professional development opportunities, and posting industry information on Twitter.