I was hoping this tool would support directionality/sequencing of nodes.
A tool I've used in the past is SequenceDiagram.org for diagraming a sequence of service calls. While a bit old-school, it's good for its purpose.
I've also used Whimsical[0] to model flows and graph-like relationships. It's got the added benefit of allowing multiple users to view and edit the same document. Whimsical is still my favorite tool right now due to flexibility, but it's a closed-source/paid product, so I'm open to alternatives.