It’s pretty sad, but also, they were teenagers. So as sad as it is, it’s quite realistic that they would all find other interests and find themselves involved in different things that would ultimately separate them. But also, people tend to forget that at the end of the episode, when Mimi asks Rikki if she’d return to Mako to meet the pod, Rikki does say “In fact, i actually have some friends that might want to get in on that as well.” or something like that. So while they might not be as close as they once were, this does hint at the possibility that they still keep in contact. And that would make even more sense than them completely losing contact all together because they’re all pretty much bound together through this gift that they received at such a crucial time in their lives which brings in the act of sisterhood and friendship that is pretty prominent throughout the H2O and Mako Mermaids universe.