Shopify Entrepreneurs Facebook Group
Started by Jon Kennedy of the small jobs platform Storetasker, this group is the largest Shopify group on Facebook. Most of the posts come from merchants, and active moderation means the discussion is kept on-topic and free of a lot of self-promotion plaguing many Facebook groups.
Shopify Partners Facebook Group
An official group for Shopify Partners, started and moderated by Shopify themselves. A good place for partners to meet and discuss technical and business topics about the Shopify platform.
Shopify Traffic Academy Facebook Group
This group managed by The Social Salesgirls is a good size. Most discussions are around marketing and increasing sales, with merchants and partners in equal measure.
Official Shopify Forums
The forums are divided up into different topic areas covering the entire spectrum of Shopify talking points. They've been around a long time so there's a wealth of searchable content if you're willing to dig.
A big plus here is that Shopify gurus and staff often jump in to answer queries. The Facebook groups sometimes feel livelier, however there's also a lot of value to using these official forums too.