It's not about keeping secrets at that level, and frankly I'm a lot less sure about my ability to evade advertisers than the CBP. Advertisers are hooked into everything, as therefore so are aggregators. That said, the aggregators and advertisers can be blocked, ignored, and spoofed. Even if you don't, they don't get physical access to your devices, they don't get to clone your HDD and keep a copy!

By contrast, the CBP doesn't have the profit motive or apparatus to keep a "file" on each of us and match that file on the fly with our testimony at the border. You don't need to fool them in some grand way, just the digital equivalent of a false bottom with a safe hidden underneath. Beyond that, share some basic crap and explain you're not that into social media, but share what you have.

They're not going to be shocked that some people have little presence, what's going to get them interested is refusing to unlock something or conveniently forgetting passwords.