I suspected for some time that some unscrupulous websites might pad their supposed user bases with fake emails and users just to make it seem like they had more users than they actually did to fool investors and make their add revenue go up. This would have to be done somewhat covertly, and take place outside the USA, if it's an American company. They just need to set up a small secret overseas slush fund to pay off people in India or wherever to set up fake emails and make fake accounts and pay them almost nothing, like a nickle per account.
Now, I won't point fingers at Twitter for doing this. But if you look at some of the larger websites out there and the literally over night meteoric rises in users from seemingly out of nowhere just before a company takes them public, or shortly before or after another company buys them out, you certainly have to wonder how much gaming is going on to inflate and pad the user numbers. And I could list a few companies I noticed this might apply to like this, but use your imagination and do a little research instead, and you will eventually see what I mean.
What never occcured to me though, based on your post, is what if they took a list of already active emails and just spoofed those to get their fake website accounts? That completely eliminates the need to also make the fake email addresses, which streamlines the process considerably.