Anyone who wants to delete their Google accounts should take the time to scrub them, first. Delete your search history, delete every file you have on Drive, delete every last email in your Gmail inbox, delete all of your YouTube history, and so on. Otherwise, all you're doing is quitting your account and leaving your profile for Google to control, which it can still use to profit from.
Google has said before that it has a profile for just about everyone, even if they've never used Google products before. That might be slightly exaggerated, but creating ghost profiles is still fairly easy to do. And any data you leave behind before you "delete" your account (which really isn't a deletion, they just deactivate and archive it) will be used exactly for those purposes. Another important step to keep Google's prying eyes out of your business is to block third party cookies. Even after all of this, Google still gets to peak into your life any time you send an email to someone who has a Gmail account or who gives Gmail access to an external account.
The same goes for Facebook. Don't just quit. There is a way to completely delete your FB account. If you do not do this, FB still owns everything you've got on your account.
Facebook is a little more stalkerish when it comes to data. There's no real way to delete your Facebook data. Once they have it, Facebook will keep your profile for all time. Facebook does not provide any meaningful way to scrub your profile. At most, you can do things like unliking business pages, but Facebook will still retain the data in their databases. Facebook knows every person who was ever on your friends list, every page you ever liked, every message you ever sent. Changing these things on the user end never changes what Facebook retains in their profile about you. Even after you fully delete (not just deactivate) your account, Facebook will continue to retain all data in their ghost profile about you, and will continue to collect data on you to expand that profile long after you've deleted your account.
You are mistaken. You can complete erase your account. Even the backup data.
How do I permanently delete my account? | Facebook Help Center | Facebook
Keep thinking that. Just because you have deleted your account does not mean Facebook deletes what they know about you. Facebook will never delete what they know about you. Never. Like Google, Facebook maintains ghost profiles of people who have never used Facebook. They will keep everything they've learned about you in their ghost file. They have become rather good at extracting information about people through secondary methods. For example, once Facebook manages to figure out your Google ID, they will suss you out by reading Google data. Deleting your Facebook account might prevent
you from recovering your account as it existed, but they aren't deleting
their access to that data.
Facebook has even become adept at connecting multiaccount users and combining them into single identities. Some time ago there was a small dust up when sex workers discovered that Facebook was serving up their clients as "people you may know" and vise versa, despite having absolutely nothing in common that would seem to be visible. In some instances, girls maintained fully independent online identities that never crossed in any ways. Separate phone numbers, separate names, separate email accounts, sometimes even separate devices that never "cross the barrier" by ever being used to log in to the
other accounts. Much of Facebook's data collection about people happens outside of Facebook. By the time someone creates a Facebook account for the very first time Facebook already knows quite a bit about you, and they are usually able to peg you to the appropriate ghost account almost instantly.