The "official" plan is for low-value transactions to happen on the Lightning Network and high-value (e.g. >$1M) transactions paying high fees to happen on-chain.
Lets assume they solve all the math challenges with the routing: The on-boarding is still a challenge as it demands one initial on-chain transaction.
If 10% of the current facebook users want to get on lightning and we can make 6 transactions per second it will take 17 months before all are on board - and this is assuming no other types of transactions (so any payment made with BTC will delay this)
