Thanks for your interest! FoundationDB was open sourced in April, so by open source standards it’s still very much the early days but there’s already a burgeoning community. I did my best to capture the current state of the project below, but I welcome input from other community members.
Obligatory plug: we’re holding our first-ever community conference, FoundationDB Summit, this December in Seattle in partnership with the CNCF / Linux Foundation. Outside of the forums if you want to learn more about FDB, meet members of the community, and get involved, FDB Summit is the place you want to be.
The project is seeing a ton of development and use, some which is easily visible and some which is not.
FDB encourages development of layers, and some of the earliest development we’ve seen in the project has been among folks developing new language-specific bindings to interact with FDB. Those live outside of the project itself. Layer-wise, it’s still early-days but my hope is that over time the community rallies between a few key layers.
Development-wise, I see bindings and FDB-specific layers as being entry points for contributors to the core itself. The project itself is incredibly well tested, so my impression is thus far some of the earliest adopters are getting pretty far without needing to make changes to the core – and have focused on layer or application-specific integrations.
Community-wise, one thing I’d like to see in the coming months is for FDB to start having regular open development meetings to triage PRs and discuss upcoming changes. We should definitely start a separate thread to dig into that topic.
Yes.
We’re currently at 46 unique contributors and growing. Pull requests are welcome. 
Yes, absolutely! Publicly, the following companies have already blogged about their production use:
Many other companies, incl some with large scale use-cases, are on currently on the road to production. I’ll let those folks chime-in when they’re ready to engage on the forums.