Well done for reaching this stage - not an easy task.
A few marketing options:
1. Give an incentive to your existing customers to promote your services. For example, you can offer them a once off discount or 1 month premium package for each customer they bring (it all depends on your services). Or, you could just ask them. Either way, this is basically free advertising, and customers are in a much better position to pursued potential customers.
2. There is no one answer - FB or Google. It depends on your product, your target audience (their age, location, profession...for example, if they're in China Google and FB are less relevant..). your budget, the conversion rate and CPA on each of these platforms...button line, the best method is to run 2 simultaneous campaigns, which would be as similar as possible, and test to see where the results are better.
I can connect you with a Google Advertising expert if you like.
Good luck
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