Gotcha. Most subreddits are averse to self-promotion, so you run the risk of a negative response from redditors. I can't speak to Facebook groups since I don't use them much.

It might be difficult to target ads to landlords on these platforms. I am not sure what targeting attributes might be available.

Maybe you could cold email/call property management companies and you might find specific people to email on LinkedIn. They manage larger number of properties and so this might be a bigger pain for them? Cold emailing and selling also makes it possible to get continuous and early feedback from engaged customers. It might not scale, but that's fine [1]

[1] http://paulgraham.com/ds.html