Here are my five favorite product management communities, online:
It's TechCrunch for new products. Product managers hang out to geek out over new products.
Product Management Handbook Newsletter
Delivered every Tuesday, this newsletter is filled with lots of great tips & advice on how to be a great PM.
LinkedIn's Product Management Networking Group
This is the largest LinkedIn group focused on product management discussions with over 80,000 members.
Quora's Product Management Topic
I second Paul Unterberg. Quora is a great resource. If you're here, you know what Quora is all about. This topic has over 120k followers.
Product Management Case Interview Practice
The last recommendation is a group I'm trying to cultivate. It's meant for folks who are getting ready for difficult product management case interviews and looking for practice partners.