If you're a digital native you probably don't need to read this. If you don't know what a digital native is, you definitely do. For our purposes, social media means digital media that enables users/members/followers to share information largely at their own discretion. We do not, therefore, include music or video sharing sites such as Google-owned YouTube. And please note - all these media are free to sign up for and use, but if you think that means they'll cost you nothing, you should probably stay away from the internet in general because you're not ready for it. Just like free-to-air TV used to, social media are selling you, to advertisers, which is why they like to find out as much about you as they can when you "Create your profile".
A note on security - if a social media site isn't already sharing your data hither and yon because you signed the small print without reading it, like everyone else has, you're best to assume they will soon. If you don't want the world, which includes the Government and potential employers, to see something, don't put it on social media. Our own Government has already shown itself to be relaxed about dipping into our private information. The UK Government has just announced it will monitor Facebook and Twitter for references to it, to which most people responded: "You mean you weren't doing that already?" After all, it is perfectly legal - if everyone else can see something why shouldn't the Government? But you have to ask; why would your Government be so interested in what you think about it?