When it comes to "free" services/products, where do you draw the line on how your personal data/activity is used/monetized?

Monetized? I don't like that. Used for the service to actually work? Then whatever it needs to work. If I have to give out my phone number to play a video game, definitely not. If I have to provide my home address for a product to ship to my door, then I don't see the issue.

How much of your data would you be comfortable letting Lemmy sell vs Reddit?

None. I hope no instance admin does this. Other than emails and IP addresses, I don't really know what kind of data we can sell that isn't publicly available already.

If Zuck treated users better, would you be more accepting of Meta monetizing your data every way possible?

I don't really know what "treated users better" is supposed to mean. If he paid me a good sum to use Instagram, sure?

When it comes to using something for free (tangible or intangible) do you accept a company selling your personal information if their practices align with what you feel is fair?

Depends on what kind of information they're collecting/selling, whether the information is anonymized, and to whom the data is being sold to (not that we'll ever really know for sure). It also depends on what I'm getting in return.