So is social media information accessible via civil subpoena? Who knows. Courts are all over the place with it.
That said, Keith Lee reviews the relevant law and links to subpoena information for all the popular social networks in his “Social Media Subpoena Guide, 2015 Edition.”
It’s a valuable update to my 2009 posts about how to subpoena Facebook and MySpace, which are probably completely out of date now (not least because MySpace is a completely different service than it was in 2009).