I would think that they're using Craigslist as a proxy for other personals -- on the other hand, I've not read the study, and the increase seems truly correlated to the addition of Craigslist personals, perhaps because of different patterns of use? Perhaps Craigslist "NSA" personals have a higher rate of "hookups" and "risky" sexual behaviour, whereas other types of dating apps don't. I'd love to see whether the data has the same effect when you look at Tinder, as at least for my generation it's similar in target uses to Craigslist personals, as opposed to something like OkCupid and other more traditional dating sites.