One of the authors here. That's correct. The spillover to other sites is not studied.

Based on an (admittedly imperfectly) related paper with Stevie Chancellor & Munmun De Choudhury [1], I would expect that the spillover to other sites results in a 1) smaller, but 2) more committed group of users.

http://comp.social.gatech.edu/papers/cscw16.thyghgapp.chance... (see Summary of Findings, page 10)