Hi. I recommend to set up proper social accounts, make them containing clean one-liner/tagline of your web app and link to the page. Then post few tweets with hashtags somehow related to the problem your product solve. Then try to follow 50-100 guys in Twitter who could be your prospectives. Try to get follow backs. Post again. Look for forums and threads where people discuss tools like yours.

To support this campaign, try to write press release and send it to bloggers and magazines popping up on your key search request, set up links with them and ask to post on some date, then buy some targeted Ads in Facebook and Google.

More aggresive you will, more prospectives you engage. The better way is to make natural links on forums and boards related to your product. Better to make it the part of viral loop or even better part of user daily workflow.