Even marriages can be extremely abusive...
The assumption that people on your friend's lists, Steam or anywhere (even just people in the same household) should be able to see your personal information, such as computer use, is a bananas assumption. It is an assumption that I'm pleased to say has failed privacy reviews at at least one company larger than Steam.
In that setting I don't think there's much "trust" happening. Certainly an average user would not think they are sharing this information with that type of person and certainly the average user believes that setting themselves to be invisible means nobody can see when they're online or any of their activity.
Besides, why would it matter if it is common? At any given point in time Steam has 20m-45m active users. 0.01% is still 2k-4.5k people. Take every single person you know by name, this is probably 10x that. This is 10x-20x the number of friends the median facebook user has. It's a very small percentage but a very large number.
We work in computing... scaling means small percentages are still big numbers.