What software management tools can a product manager use for doing Project Estimation?
You can try using Aha Roadmaps which is mostly used for product strategy and road mapping.
We haven't seen a high return in engaging with social media communities (FB, Reddit, IO Games). FB and Reddit in particular had little - if any - return on the day of the launch, and the Slack groups weren't much better. If you haven't joined any of those groups by now, good luck. Some of those groups can take months to get in because they aren't actively moderated.