F5Bot is a free service that emails you when your selected keywords are mentioned on Reddit, Hacker News, or Lobsters. Use it to monitor your brand, your projects, or just topics that you're interested in.
Seems to work very well too! Just got an alert that someone shared one of @plausible blog posts on Hacker News (I've added our domain name to be monitored) and I got the alert within a couple of minutes of it being posted there. Very cool!