Since 1950s, media content analysis has turned into a prestigious research methodology in both mass communication studies and social sciences. As you have rightly observed, regressional analysis of the content has been the norm in content analysis operation. I refer you to the links below, which can hopefully provide you with the information you are looking for.
I am not aware of academic studies quantifying social media posts in terms of quality. But there is abundant advice in the popular media about how to construct strong tweets. Things like strong adjectives and descriptive language, and calls to action.
The blog linked below is one of the best sources I have found about how to use social media effectively. If you peruse this blog, I think you will be able to construct an evaluation rubric.