ResearchGate
ResearchGate is a social networking site for scientists and researchers to share papers, ask and answer questions, and find collaborators.
It was launched in 2008 and has currently about 4 million members.
SSRN
Social Science Research Network (SSRN) is a website devoted to the dissemination of scholarly research in the social sciences and humanities.
SSRN is a subject repository for several disciplines. Some publications are free of charge, others are available for a fee.
Academia.edu
"Academics use Academia.edu to share their research, monitor deep analytics around the impact of their research, and track the research of academics they follow."
The site was launched in September 2008 and has currently almost 10 million members.
LinkedIn is a social networking website for people in professional occupations.
It was launched in 2003 and has currently more than 300 million users.