You can certainly pay for eyeballs, the question is whether FB ads are giving users with a retention level high enough to justify the advertising expense. Users from online ads tend to much lower quality than those that come from organic traffic, I would not expect your organic retention numbers to hold.
I've said why its not growing organically many times. Most businesses have no viral coefficient, so organic growth is linear and slow. Some consumer focused tech busnesses are the exception, not the rules.
And yes, all ad spend should be measured against true LTV.
You can certainly pay for eyeballs, the question is whether FB ads are giving users with a retention level high enough to justify the advertising expense. Users from online ads tend to much lower quality than those that come from organic traffic, I would not expect your organic retention numbers to hold.