It's a rule of thumb. No, not every costless product is monetizing your use of it. FOSS is especially counter to this rule. In the case of web-based tools that

It's a rule of thumb. No, not every costless product is monetizing your use of it. FOSS is especially counter to this rule. In the case of web-based tools that are owned and operated by big advertising companies, I think it's a fairly safe bet. As always, think critically about each instance and consider the implications of the software and products you use.