If you can find news and other interesting items related to the web app, set up a Twitter, Facebook group, blog, Tumblr, etc. and shovel a couple pieces of content on there a day. Search Twitter and follow everyone talking about that niche, retweet their stuff, do the follow Friday posts, etc. (Take a look at http://challenge.co for more ideas on testing the market first, which you can then use to promote your app.)