As I, and many others, have said before: what if you tell them you don't have any such accounts? And I don't mean: what if you actually don't have any accounts.
What will they do if you just tell them you don't have any details to give?
This is the big question because it undermines the whole concept. How do you know they are lying? Yes, if you have a Facebook account in your name with your photo then that is something that can be searched and found but what if you don't have any directly identifying information visible in those accounts?
At that point, the border control staff have two options: believe you and let you though or don't believe you and single you out for more rigorous screening. The latter amounts to discrimination, though, unfortunately, not of any kind that you are protected from.
It's a thought that crosses my mind when I see police. Not that I have ever had any negative interactions with them, but I do not carry any photographic ID day-to-day. I just don't.
So what happens if I am stopped by a police officer and asked for identification? Australian police have the authority to do this and I have an obligation to produce my ID when asked but I have no obligation to either own such ID nor to carry it on my person at all time.
And that's the problem (one of them!) here as well - border staff will have the authority to request details that you are under no obligation to actually have in the first place.