You got 6 invitations to your service for a $150 spend. So that's a $25 cost per invite. What's the total lifetime value of a landlord to you? If it's more than $25, that's a win right? You have to discuss these results using math in order to say win/fail.
Also you're right that spending more and playing with the headlines day to day is a good idea. (I hear @patio11 in my head shouting "A/B testing!") The $25 acquisition cost could drop to $10 or $5 with the right message wording and trying different subreddits.
Lastly, I always shudder to hear about $100 FB ad campaigns people run and declare them to be a waste of time. As someone else said, it is a bit like golf. You have to play it a few times to get a sense of if you're good at it. The first time out is not a sufficient test.