I don't know if this is the place for this question, but I'll go for it.
Websites like facebook, twitter and so on and so forth, have big storage space as there servers. Why does it need to be so big?
If I (I being a company not a home user for this example) have a website, except for the content I put in, what other information is stored on my server?
Not unless you specifically allow uploads of material (as Facebook and Twitter do). So, for example, if you run a forum on your site then data will be added. But it would have to be very successful for this to amount to a significant amount of storage.