But I think the crucial point is that if you want to fight loneliness, it is difficult to do that if the lonely one is made to think that he is now getting something because he is lonely. That will instead make him feel worse. It would be much better to tell to the lonely one that hey, just you, I would need your help. (Complete amateur psychologist writing here)
Scoring has its challenges, but somehow I think there should be a way to flag people that do not behave decently. Maybe just two grade scoring with wording that makes it clear that the lower grade is applicable only if the person has e.g. threatened with violence or completely disregarded what was agreed (being a bit late or politely cancelling in the last minute is not sufficient)
And for the second part, I just thought that it would be enough to get a notification on your phone that someone nearby could use some help that you can give. Not probably a perfect solution that saves every lonely soul, but if even a few, that would not be bad. And of course, maybe also some of the not so lonely ones get to receive or give some help.
I am open to discuss about this further, though, if there are people interested in taking a lead. (Ideally I would see this built as a open source client on top of some distributed data store and not a for-profit thing, but that probably is way too idealistic)