Academic social networks allow scientists to share their personal profiles including lists of outputs, follow each other to be notified of new papers, request papers from an author and upload papers that can be downloaded directly. These networks are a good way for young career scientists and scientists from developing countries to become part of the scientific community in their field.
All networks offer these opportunities, but they all have their strong points.
ResearchGate is arguably the largest network and papers uploaded there are particularly well covered in Google Scholar.
Academia.edu has the best coverage of profile information in the general Google search engine.
Scholarmate has strong links with the Chinese academic community and offers recommender services for journals and collaboration based on text-mining of the contents of the network.
SSRN is a network for the Social and Behavioural Sciences.
If you upload papers you should be aware of copyright restrictions.
The above mentioned services are prolific senders of e-mail but they offer options to turn off unwanted e-mail notifications. They are sometimes criticized for sending e-mails seemingly on your behalf to your co-authors to invite them and confirm their co-authorship (and in the process signing them up to their service).