Everything online could last forever. Even old versions of websites could last forever. I used the Wayback Machine to view an old version of the Girl Scouts website and compared it to the more recent version. You might be familiar with Snapchat. Snapchat's privacy policy says that after 30 days, your messages are completely deleted, even from Snapchat's server. But someone could easily save that message by taking a screen photo of your chat. Then, they could post it online or save it to their photos, and then it really will last forever. So, everything could last forever online. That is why you should always think before you post, even if it will be 'deleted' by Snapchat or another company.
In some places. In Europe and in Calfornia, you can request that the comany let you look through all of the information they have about you, and you can even ask them to delete it. In Europe, that is called 'The Right to be Forgotten'. In California, that is called the California Consumer Privacy Act, which only applies to people in California. Just to make it easier, some companies allow everyone to look through and delete their data.
Yes, they probably know some things about you. Even if you don't have an account, Facebook probably has a seceret 'shadow account' for you. They get this information about you from people you know. No, your friends and family are not selling your information to Facebook. When you create an account, Facebook asks you is you want to allow access to your contacts. Most people say yes. When Facebook goes into your contacts, it looks for someone who does not have a Facebook account. That's you! They pull off your name, email, and other information, maybe even where you live, and put that into your 'shadow account'. If your friend posts something about you, say, how jealous she was of your vacation to Hawaii, Facebook will put that in your 'shadow account' too. And the scary thing is, you won't even know it's there. How does this fit in the topic of information lasting forever? Your 'shadow account' will last forever and Facebook will probably keep it forever.