Having an online presence is definitely helpful for getting jobs and finding other opportunities. You are probably online already, so you might as well have some control over your image. Google yourself and see what you find. Then decide what you want your online presence to look like and use some of the tools in this guide to help you.
ORCID, an open, non-profit, community-based effort, can provide you with a persistent digital identifier that distinguishes you from every other researcher.
Academia.edu - A Social Network for Academics
Best network for academics. Ranks high in google searches
Business-oriented, but widely used; ranks high in google searches.
Vitae (Chronicle of Higher Education)
Citation software that allows collaboration and document sharing.
Citation software and social network.
SSRN - Social Science Research Network
Rapid disseminator of social science research. Some documents are openly available; some require paid membership.