Gitter co-founder here: we use neo4j for suggesting rooms.

So, if you're in room A and room B, and most people in those two rooms are also in room C, then we suggest that you join room C.

As I mentioned elsewhere, we also use some non-neo4j based methods for suggestions (including your GitHub graph). One of the problems we've had with neo4j is that we haven't been able to make it scale. It frequently burns up from being overloaded.

This is almost certainly down to the way we use neo4j but at some point I'd like to ditch it for a clustered suggestion algo that uses batch processes to cluster rooms together.