All iphones and smart phones have data plans. This means that you are using data anytime you are using celltowers and NOT wifi for anything which is not phone calls. So this means using an app like facebook or instagram or twitter over celltowers is connecting to the internet and using data passing it back and forth. Videos really use ALOT (for example youtube).
To stop this you can turn off cellular in your settings and turn wifi on to make sure you are only using wifi.
Text doesnt normally mean much when it comes to data, thats really "tiny" sending text around. Unless you were attaching pictures to every text you send then that would make a difference but data usage is normally social media apps that consume the most, or anything streaming music or video.