Exactly....but Facebook doesn't have state-issued uniforms, arrest powers, and weapons. Facebook doesn't, as a matter of course, have the legitimate ability to stop and detain people.
Yeah, it was voluntary...but when was the last time you saw a voluntary road block? When you're driving down the road and you see a checkpoint do you say "Well, this is optional." in your mind? Or do you slow down and get your license and registration ready?
People upset about the cops conducting a road block to ask people for DNA samples are right on the money.
There are things that police officers in uniform (conveying the authority granted to them by the state) simply should not do. We're not talking about a knock-and-talk in the course of investigating a crime, or even urging kids to stay off of drugs in a school assembly. Armed agents of the government stopping people and asking for their DNA is well into the realm of things that just shouldn't happen.