LinkedIn is generally considered the social network for adults and professionals.
Per Visual.ly (May 2013):
There are a variety of strategies of how people and businesses participate on LinkedIn. What is consistent, though, is the need to be there.
LinkedIn mastered the degrees of connection (Sorry, Kevin Bacon!). Who is in your network? Did you realize that someone you know used to work with someone you've been wanting to meet?
Is your company looking to hire talent? LinkedIn has become a premier spot for recruitment.
Referrals? LinkedIn has a built-in set of tools to get recommendations.
Whether for your personal brand or for promoting a company image, LinkedIn is a valuable resource which also carries strong Search Engine Optimization results.