How to save data on my iPhone?

I downloaded Facebook, Instagram, Twitter and Tumblr.
Do these apps use a lot of data? Do I have to turn off the notifications?
Help please, I don't know anything about "data"

Settings > General > Cellular >

Once you're there, you can turn data off completely when you don't want it to be used.
To save data, turn off data roaming.

As for updates, you can always check with data tracking apps. Depending on how much it updates, I highly doubt you can go over the limit just from status updates.

If you're on a limit, the Data Usage app for 99cents is a good investment.
(Note* if you get the Data Usage app, it does not actively update, you must check to see where you are at)

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