They seem to use "social media website" interchangeably with the actual apps in question, and their methodology writeup doesn't mention the actual questions they asked in their telephone surveys.
I wouldn't be surprised at all if more people used the Pinterest website more than the Twitter website, as the latter seems more oriented to being a website. It would appear that Twitter has way more users overall as compared to Pinterest though.
Pinterest: "50 million unique monthly users worldwide" [1]
Twitter: 117 million monthly active tweeting users [2]
So headline should technically be "Pinterest website passed Twitter website in popularity ..."[0] http://pewinternet.org/Reports/2013/Social-Media-Update/Main...
[1] http://allthingsd.com/20131023/pinterest-does-another-massiv...
[2] http://blog.peerreach.com/2013/11/4-ways-how-twitter-can-kee...