The 'people' referred to are trolls, stalkers, or other unpleasant or abusive individuals. The dating site has set up various filters and screening systems to try to ensure that these people can't join. However, it's impossible to ensure this 100%. Maybe someone is banned for abusive behaviour, but then they manage to join again, by using a different name and IP address. If they manage to do this, they 'slip through the cracks'. Imagine you put up a wall to stop insects coming into a room. This will keep out most of them, but a few of them may find a crack in the wall and slip into the room anyway. That's what this idiom means. It's a common and useful expression.