Pay them.

Seriously. In El Salvador, the $30 worth of bitcoin could not be cashed out at the ATMs. They had to spend it. That ended up creating a few million users. But most of them were one and done. Most haven't touched their Chivo wallets since.

So subsidizing adoption with giveaways won't work. But that's not the only way they could be paid in bitcoin. In a circular economy, vendors, merchants, and workers (sometimes) earn in bitcoin. They are more likely to then spend in bitcoin. Which results in others who then are earning in bitcoin. The bitcoin persist (to some degree) within the circle, rather than getting cashed out.

But circular economies don't just form organically, quickly. They can be accelerated with incentives (e.g., discount for paying in bitcoin). They can be promoted in a number of ways. But who subsidizes that promotion? Payment processors? That's not in their budget, when they only take 1% of the revenue (versus the 2% to 4% taken when the payment card industry e.g., Visa/Mastercard).

So for it to happen, it will likely be from the bitcoin plebs patronizing the few merchants that do accept bitcoin, which then gives incentive for other merchants wanting your business to follow suit. How do we communicate to a merchant that we would like to pay them in bitcoin? Well, gift cards bought with bitcoin are a conversation starter. Each time you pay with the gift card, let the merchant know that you had to buy the gift card with bitcoin, and that you'ld rather pay them directly. When they hear that from a few different customers, they might start to look into it further, rather than just dismissing it as being too niche.

For a faster path to 1 billion additional people "into bitcoin", that's likely going to occur first from those people "into bitcoin" for price speculation.