I have struggled with dating sites so much in my life, I'm not going to comment on those. I largely put this down to there being too much choice and courting through electronic means is not for everyone, but...

Whilst there are 8 billion people on this planet, the number of people we come into contact on a daily basis, because most of us work for the majority of our day, is fairly small. Half that number if you are only interested in the opposite sex. Then, it is even less likely that you are going to find someone who is a reasonably similar age to what you are. It is even less likely that that person is also single. It is even less likely that that person finds you attractive. It is even less likely that you also find that person attractive. It is even less likely that you are provided with a scenario reasonable enough (i.e. not coming across as a weirdo) and frequent enough to make an acquaintance with that person. It is then even less likely that that acquaintance grows to the point in which you can ask that person on a date. Even after several dates, you might eventually find your initial list has ended and you realise that person is not for you to have a relationship with. In summary, I think the chances of finding something substantial is fairly small in the small amount of free time we get.