Prove that Steam collects data about your system, except for hardware statistics that it asks you want to send. Being the first platform for gaming, I think that Steam gets audited by a lot of people quite often; despite that, I have heard nothing about Steam spying on its users.

The only "invasion of privacy" found was in VAC, which uses security through obscurity to deceive cheaters. When somebody got concerned about that, Gabe Newell immediately explained publicly the security mechanism, letting everybody with basic IT knowledge deduce that Valve has thought this quite thoroughly for privacy.

Last time I refused to participate in the Steam hardware survey, it was because the description of the data sent in the survey included "software installed on the system." If you use anything other than Steam on your system, that's not really acceptable. AFAIK, though, that is only if you join the Steam hardware survey.