Beyond that I think there's also a first principles approach one can take. Not only were these services addictive, but so was Facebook (and other similar services) long before 'the algorithm' was a thing. The core interaction is interpersonal, and people simply want to see what other people are saying. We could define that as FOMO, entertainment, or whatever else - but I think this is the shared common core.
Even on this site, there are many people who are clearly addicted - myself probably amongst them. Yet there's not even a notification of when something you've written has been responded to, let alone some guiding algorithm. Even having to jump through hurdles to achieve interpersonal dialogue, it is still quite addictive.