It's hard to find new referrals, but once new features are added, there are good chances many of those old users will come back in fresh new accounts, because it's common to forget passwords and stop using old email addresses along the years, so they have no choice besides creating a new account through a new referral link.
Furthermore, there are always potential referrals around: many people who were children when freebitco.in was firstly launched are developing their interest for crypto currency universe only now, so it's a perfect time to refer those youngsters to the site.
I think it's harder to refer people now because the old platforms we were used to share links aren't efficient like before anymore. I mean mainly PTC sites which are completely dead and facebook which is infested with scam earning offers, so no serious person pays attention to what is posted there anymore.
Hmm, I will not create a new account as on most sites it is not allowed to have multiple accounts under the same IP. I think a lot of people use an old email adress that they are already using multiple years (I am using mine already 15 years). For a new site, you can still get a lot of referrals but the platforms have indeed changed, because as you mention, who still uses PTC sites? I stopped using them a long time ago.