How does Facebook, Twitter etc use MB on my iphone?

[Maci Pascal]

I would check my usage under settings on my iphone4s. And apps such as Youtube, Twitter, and Facebook would be using MB. I just wanna know what im doing that makes it use MB. and how i can decrease the amount. Like when i use twitter, everytime i open it does it use MB? Or everytime i watch a youtube video? Help !
Ok soo youre saying that everytime i use facebook, open twitter, or watch a youtube video, im using data. (Mb, GB on my phone) right?& on my phone it shows the amount available&used under settings<usage right?
**How do i get more storage space on my phone, by doing something about my facebook, twitter, youtube, etc. apps?** is there anyway i can use less data?