Earlier quoted context omitted.
That's expected given Twitter leans more towards text content. The proportion of Tumblr posts that are images or video has always been much higher than Twitter's. Twitter didn't even have built-in photo posts until mid 2011, and even then it was through a third-party integration with Photobucket for the first year. Images weren't shown inline in the Twitter timeline feed until late 2013! Meanwhile direct video upload…
What I'm trying to say is, to me, that just generally sounds like a pretty large share of adult content no matter how you shake it. I could be over-estimating, but I'd assume it outpaces all of the other major social media platforms and most websites of similar magnitude in general that do allow pornographic content but are not explicitly porn sites.
On the web, similar situation for DeviantArt and a lot of other sites.