Sure, they can have the accounts i don't use.
Which makes me think that it'll end up leading to a lot of hassle for a lot of asocial people. E.g. my Facebook account only gets logged into 1-2 times a year. Would they believe it's my real one, or suspect I was doing what you're suggesting.
But telling the immigration official still is optional, since we already know who you are, what you buy and what por- uhm, websites, you like. (discretely fist-bumps Goomazbook) Welcome to this great nation!