You can split in 2 strategies long term and short term. Short term you need ads.

Ads via Google work because you get immediate response, trouble is if your not well acquainted with SEM you may overspend do to the lack of bad landing pages, unuseful keywords and just low conversion rates. But if done properly you will have a machine with good ROI.

Facebook has now a deeper level of interests, from special demographics to campaign audiences that can be built from previous lists you may have. Lets say you have a landing page where you said soon to launch and collected emails. You could later ran ads to those users via fb or use their lookalike feature to relate to users that may have the same patterns or behavior and thus promote to a bigger list.

Long term you could focus on inbound content, videos, lists that add up to content that people may search if you have a good understanding of what keywords are trending and what is their volume of search. Also how hard it is to get your content to rank via SEO based on what your competition is currently doing.

happy to get on a call if you need more guidance.