LinkedIn is a social networking site with a focus on job recruiting. However, it's not all business. Many scholars use LinkedIn to show off their academic service. If you're investigating a non-traditional career, or even if you just want to ensure discovery of your CV by university administrators and support staff, LinkedIn can be an important complement to your other social networking activities. Even if you don't use it regularly, make sure you update big career events such as teaching posts, fellowships and grants.
Academia.edu is a Facebook-like social networking platform for scholars. It allows people to search by name, research interests and universities. The site is highly visited and prominent in Google searches. Academia.edu allows you to update your status, upload your academic papers (including unpublished drafts), list your research interests and follow other scholars with similar areas of research. This site also provides great assessment tools that allow you to be notified when someone views your profile or papers.
With more than 6 million researchers, ResearchGate is the leading social networking site for scientists and an increasing number of scholars in humanities and social sciences. It offers the same functionality as Academia.edu: share publications, get statistics about views and downloads of your research; collaborate with peers, etc.
Mendeley is a social reference management site. You may already be using it to store your citations and help keep track of your research, but it also has some great social features. You can list or upload your research publications, provide a brief academic CV, biographical information, and participate in a group. Mendeley groups allow you to share and discover new research in your field.