The best alternatives to Little Birdie are Notify, F5Bot, and Karmalert. If these 3 options don't work for you, we've listed a few more alternatives below.
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AssemblyAI— Speech-to-Text API with superhuman accuracy
Whenever your Startup gets mentioned on Twitter, Facebook, Instagram, Youtube, Google+, Blogs, Forums, News, Reddit, Medium, Hacker News and Product Hunt. Get Notified on Slack or HipChat.
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.