Snapchat is not the only problem here, but it is a problem.
If they can’t guarantee their recommendations are clean, they shouldn’t be offering recommendations. Even to adults. Let people find other accounts to connect to for themselves, or by consulting some third party’s curated list.
If not offering recommendations destroys Snapchat’s business model, so be it. The world will continue on without them.
It really is that simple.
Using buggy code (because all nontrivial code is buggy) to offer recommendations only happens because these companies are cheap and lazy. They need to be forced to take responsibility where it’s appropriate. This does not mean that they should be liable for the identity of posters on their network or the content of individual posts—I agree that expecting them to control that is unrealistic—but all curation algorithms are created by them and are completely under their control. They can provide simple sorts based on data visible to all users, or leave things to spread externally by word of mouth. Anything beyond that should require human verification, because black box algorithms demonstrably do not make good choices.
It’s the same thing as the recent Air Canada chatbot case: the company is responsible for errors made by its software, to about the same extent as it is responsible for errors made by its employees. If a human working for Snapchat had directed “C.O.” to the paedophile’s account, would you consider Snapchat to be liable (for hiring the kind of person who would do that, if nothing else)?
Then, in my opinion, you would have failed to perform due diligence. Even if you’d thought C.O. was an adult, suggesting a woman strike up a private conversation with a man neither of you know is always something that deserves a second look (dating sites excepted), because the potential for harm is regrettably high.