On Pornhubs side, it's essentially a steal. They could very well foster a good atmosphere when it comes to porn and then offer through their mindgeek advertising platform access to Tumblr for their marketing affiliates.
These marketing affiliates spend ALOT of money on ads and it's very profitable if you know what you are doing.
If they started to get their own studios or their affiliates in creating films (actresses) in the recording space to use Tumblr to blog and not use twitter. Again, this would drive engagement by quite a margin.
There is so many things Pornhub could do here to make hundreds of millions off the back of Tumblr and they have the management to do it.
[0]: https://www.businessinsider.com/verizon-looking-sell-tumblr-...