You can absolutely dominate with this (assuming your web app is excellent) by doing Facebook Ads. I recommend right hand ads that are targeted to people interested in various SEO keywords, groups and pages.

We do this for clients all the time and by far, Facebook is the absolute best (and most affordable) way to market products and services like this right now. The targeting, especially using the Power Editor, is ridiculously good.

I'd recommend testing 50 different ads and 2 methods:
1. Landing page that is on your Facebook page as a tab
2. Landing page on your actual website.

This works really well right now, even at lower ad budgets.