If your goal is create one shots that get folks interested in D&D, then I encourage you to look to /r/LFG on reddit.
Yes, reddit in general has a bad reputation, but I find that LFG is a decent community that has quite a lot of web traffic. You can also post in /r/criticalrole - that community is really a fan of Matt Mercer's storytelling, so you can bet that they are all interested in narrative roleplaying, but most of them are shy when it comes to trying out games.
On Roll20, you've really just got forums and the game-search system.