Remember, there are clear legal restrictions on what you can and can't do when it comes to accessing or tracking someone's phone.Make sure you're aware of them before you do anything that might get you into trouble with the law.
If nothing else works (or you don't have the time to conduct an exhaustive search that may or may not be conclusive), consider hiring a private invesitgator.
They could also be using information you simply wouldn’t know.
In that case, an option is to create your own account on one of these sites and then search for their physical attributes (age, hair color, etc) and location to see if you find a matching profile.
Whether you fall in category A or B, you should never be discouraged.
The mobile market has a variety of apps designed to improve your odds of scoring on a consistent basis. Take a look at 11 Apps That Will Actually Get You Laid.
A fake number will work for sites that store credit card information to either charge you later or ask you to upgrade.