If you use services such as Gmail and Google Maps, or Meta products such as Facebook, the companies know an alarming amount about you.
This can extend to a second-by-second GPS map of where you go every day, everything you look at online, who your friends are, to things you do in apps and sites that aren't even owned by Google or Facebook.
Thankfully, it's fairly easy to take control of what Google and Facebook know – and it's well worth doing, says privacy expert Rachel Jones, founder of brand protection company SnapDragon Monitoring.
"Today our privacy has been exposed to tech giants like Google and Facebook in ways few people can truly understand," Jones told Yahoo News UK. "Every click is monitored, every website visit tracked, and every photo is catalogued to learn more about our personal lives, our likes, and our behaviours.
"This intrusion is promoted as a better way to tailor content to users' preferences, but privacy is its casualty."
Jones warned that allowing your personal information to be stored by companies could put you at risk of your information being used to 'train' artificial intelligence (AI) – or even being accessed by cybercriminals.
Jones said: "However, there are things you can do to mitigate the concerns. Be careful what you post online, only use sites that you trust, use privacy-focused software and search engines like Firefox and Duck Duck Go.
"Being vigilant, and prepared, can protect your privacy in an online world where being 'ex-directory' often feels like a luxury of the past."
How to delete everything Google knows about you
Google knows an alarming amount of your personal information – particularly if you use an Android device or services such as Google Photos or Gmail.
Every time you search, speak to Google Assistant, or perform actions in apps linked to your Google account, it's stored online.
Enabling Google Maps on your device (depending on the options you select) also probably means that Google has a record of everywhere you have been, along with timings accurate to the minute. You can see your location history here.
Thankfully, it's now considerably easier to take control of this data.
There's a single My Activity page that brings together all the different data sources Google has on you – web and app data, which includes searches and actions in apps linked to your Google account, location data and YouTube history. You can visit that page here.
Clicking on one of the three menu items allows you to delete your entire history as stored by Google, or set it to auto-delete on a timer, or set it to never record these details in future.
It's worth noting that your Chrome browser history is separate from this page, so to get rid of that, open Chrome, click the three dots in the top right (on desktop) click 'history' and select 'clear browsing data', then 'all time'.
How to delete everything Facebook knows about you
Facebook doesn't simply collect information about who you are friends with, what you 'like' and what you post on the site – it also collects information on a huge amount of your online activity while you are logged into Facebook.
Thankfully, it's now easy to see all of this information via Facebook's 'your information' page.
From this page, you can manually delete items from your Facebook history – for example, posts or interactions with other users.
You can also see other information under 'personal information', 'connections', 'logged information' (which includes your location and search history) plus 'apps and websites off Facebook', which includes information Facebook receives from other sites.
Each of the sub-menus offers tools to manage your information ('ads information' also offers a list of advertisers who have targeted you).
It's well worth visiting 'apps and websites off Facebook' and 'off-Facebook activity', from where you can limit what information other sites share with Facebook.
Choosing this option means that Facebook will no longer track you across the web, so it's worth doing.
Facebook's tools allow you to manage and delete your activity manually, which is a great way to get rid of embarrassing old posts or 'likes'.
If you want to get rid of everything in one go, Facebook's solution is to delete your account – with information here on how to do so.