BitBucket doesn't have a large OSS community, but it does have a lot of larger business that need Jira and Confluence integration. So it could be worth it, just the user base isn't as visible as GH.
Definitely something to think about then, since I think larger businesses are the sweet spot for Reviewable and that's where most of the growth has been coming from in the last couple years. Still, forking the codebase _really_ doesn't sound appealing...
Abstractions are sometimes really good for understanding the business. But yeah comes with a price tag and potential mismatch to the real niche (like GitHub add on)
BTW, does anyone where to get better-than-anecdotal data on userbase size and composition for GitHub, BitBucket, and GitLab? And, ideally, growth trends?
The GitLab number tallies with Emily's answer, which gives me some moderate confidence that these numbers are in the right neighborhood. I'm also going to assume that the users-vs-teams numbers are all in roughly the same proportion.
Based one these numbers it doesn't make much sense to target BitBucket unless either 1) they're growing much faster than GitHub (unlikely) or 2) their customers are a much better fit for Reviewable (possible, but it would have to be a truly significant difference in userbase composition). Targeting GitLab doesn't appear to make sense at all (as a business), though perhaps they're growing fast.
It's hard to compare, since Reviewable is an OAuth app and hence needs an install per-user rather than per-org. However, on the page you linked I see a couple apps topping out at 6-7k installs, and most of the rest are in the low hundreds. This doesn't look particularly impressive to me...
Community Advocate for GitLab here. We can't share that data publicly (also because it's hard to track since many are on open source core), but we do have over 100k orgs using GitLab