Twitter connects you to your friends via instant short blog messages. May be you can consider it as analog to all instant messaging apps and all networking apps. And yes, it's core feature is to social networking. It helps you to share some strange things like "hey, I found an apple in my hat" with people, who may answer "so cuuute".
Facebook app has the only feature - to provide with social networking. Compared to analogs like WeChat and WhatsApp, this app is more intuitive. In all aspects, related to social networking, Facebook app is more useful than competitors. There is just one thing that can make you nervous - you cannot easily take a link from Facebook and share it on WhatsApp.
This application was made to ease you the process of taking payments on-the-go - it's a main feature of an app. Yes, you don't need any other device, because you will use only your iPad or smartphone. To take a payment you will need a swiper or just a camera of your smartphone or tablet. GoPayment is very easy in use and can sync with other apps
As you may know, you cannot use iPhone as a USB device, while sometimes you may need such functionality. And OrganiDoc app is solution. Its key feature is to turn your iPhone or iPad into a USB flash disk. It will help you easily work with your files on your device and open them at anytime, without depending on different syncing features of other software.
The Currency Converter app is to help you be on-touch with currency rates. The main feature - to convert currency (thanks cap!). Currency Converter gives you exchange rates between 180 currencies and 4 metals. This app uses the Interbank rate, or choose a percentage add-on to better approximate the actual rates.
WeChat is the one of social networking apps that provides you with instant messaging. This app more or less simple in use. It has some additional functionalities like voice chat to ease the messaging while on the go. But the main feature is anyway the instant messaging, as something people use to for quick communication with friends .
This app lives in a huge number of smartphones around the globe. This app provides you with instant messaging as a core feature and some additional functionality. In fact, WhatsApp may become your universal tool for comunications on the go. But be patient while installing - it will take time to start usage:)