This blog post by Lou Dubois writing for Inc. outlines some key elements of a traditional media release and gives guidance on how to adapt these to social media.
It also touches on how to measure engagement.
Dubois, L. (2010). 'How to write a social media press release'. Inc.com. Retrieved from: https://www.inc.com/guides/2010/11/how-to-write-a-social-media-press-release.html