The most popular commercial scholarly profile services-- Researchgate.net and Academia.edu-- have recently been hit with a spate of 'takedown notices' by commercial publishers. Under the law, websites receiving a legitimate copyright takedown notice must remove access to the infringing material.

Add this to the fact that both Academia.edu and Researchgate.net are commercial services, dedicated to monetizing your content, and many academics are rightly concerned about using these services to host their material.

SSRN, Social Science Research Network https://www.ssrn.com/ is a specialized repository that has been purchased by a commercial publisher (by Elsevier, the owners of ScienceDirect) and has plans to allow draft articles to be posted on SSRN and submitted for review to a journal at the same time.