Interfacer - Collection of more than 200+ free design resources
SocialSizes - Sketch templates for sharing images & videos on social media
Facebook Design Resources - A collection of free resources made by designers at Facebook
Social Templates Bundle for Instagram - Large bundle of 240 posts & stories templates for Instagram
Freebiesbug - Collection of the best free web design resources.
Video Templates By VEED.IO - Professional & editable online video templates for social