Why can a Wi-Fi user use the Internet even though I have my Social WiFi splash page set up and my network's walled garden configured correctly?
In order to allow users to connect to your Wi-Fi through your splash page, Turnstyle may allow connections to Facebook, Twitter, and Google. In order for these connections to happen, they are exceptions to your walled garden.
That means a Wi-Fi user at your venue may access those websites (Facebook, Twitter, and Google) without going through the splash page. However, as soon as they visit a website that isn't Google, Facebook, or Twitter, the splash page will come up and they will have to connect to it to use your Wi-Fi network.
Note: For a solution that forces the Captive Portal on Android devices please see this article.