An evil data scientist would put almost exclusively users on your screen who you are interested in but they are not interested in you to boost your spendings and would mix a small portion of good bidirectional matches just to make his purpose less obvious. Fortunately evil data scientists does not exist... I mean I definitely wasn't paid to do anything like that. Never.
Tinder does not hold anything even remotely resembling a monopoly (no, Match does not own every dating app, just many of them) on the dating app space, we need to stop throwing that word around so casually. You're diluting the concept by trying to apply it here, which will lessen its impact when a real monopoly comes along and actually tries to control a market (e.g. Microsoft and how it's handling Edge right now).
The world you're looking for is China, where without WeChat you may as well not exist.
And on the meaning of monopoly, I believe it is fair to describe the "network affect" as a monopoly. The historical example of the US rail comes to mind, the market had several rail companies at the time, but each had monopoly over certain routes, if you wanted to go from place A to place B you maybe only had one choice. The fact that other rail lines existed to places you didn't want to go doesn't mean that it wasn't a monopoly. The same way, those tech companies may have monopoly over the route from person A to person B.
True, your 'island ferry example' is a monopoly. It is also not at all analogous to this situation and Tinder in general.
I'm no expert, but I don't think you understand what the 'network affect (sip)' is, as it does not apply to US Railways at all, and is a relatively new term that only arose in the 1970s. That aside, your example doesn't survive further scrutiny because railways were often the only practical way of moving between two generally public places — cities or towns. With Tinder though, they are providing you access to their own private network of users. Your example is like saying the monorail in disney is a monopoly, but that doesn't make any sense, since it's for transporting you within their own park.
Lastly, Tinder is not the one and only way to meet people. Yes, they have are of the most widely-used companies in the space. But there are others. And, you know, people can still meet in real life — through friends, at work, at activities or interest-based groups. On other social media like Twitter, et al.
This is true for brick and morter/mom and pop businesses. It's true for computer programs. Basically any system. It is not an indicator for a monopoly.
Every system that survives, survives because it generates positive value for most people, no? Are you telling me that every business that isn't a monopoly only generates positive value and there's no negative value generated for a minority of customers?
That sure is a monopoly.