If you asked me in 2009, a GitHub would be enough for a developer's portfolio in the context of an open-source developer. Given the mainstream status of Twitter in referencing tweets as "evidence" in online articles or participation in conferences/meetups, it may be wiser to be on Twitter to market yourself as a dev, rather than the rest of the alternatives.

Facebook, LinkedIn, Instagram and Snapchat serve no utility for a single developer but instead benefits better for an organisation.