M1 MacBook users - can you tell me how much memory is used

Don't worry about it. Check the memory pressure bar, and if it is green, then this indicates your MacBook is using its memory correctly. If you visit: https://support.apple.com/en-gb/guide/activity-monitor/actmntr1004/mac this explains how you can check this for the operating system you are using. It is known that an operating system will make sure your computer is making the most use of its given RAM. It would be wasted memory if the operating system wasn't trying to use all of its memory.

My Macbook M1 base model (8GB RAM and 256 SSD) uses 471MB of memory from Facebook (currently). In total, Safari is currently using 547MB. Could you specify what you actually want to know this for? It may help me or others, giving a more comprehensive answer.

Just remember that the M1 chip is going to be much more efficient than previous chips. There is a load of videos on comparisons out there on YouTube.

Ah, the message you are seeing is simply Safari telling you that the web page you are using uses a lot of your system's memory. However, it does not indicate there is an issue with the amount of memory your machine has. I had a similar problem with my M1 MacBook the other day when using Zoom on Safari to watch a recorded video back. Apple hasn't given us an option to turn this off.

In short, there isn't anything wrong with your memory, and it is just a useful yet annoying feature you can't switch off. You could use another browser such as Firefox which doesn't have this feature.

Example. The memory bar on my MacBook is great, and this indicates the MacBook is handling memory usage well. I have a similar 'Memory Used' to you. But my MacBook is still running very fast and efficient.