Hi, I was thinking that the sum of local EA group facebook numbers might serve as a good proxy for the size of the EA movement per se.
I am part of the local team building an EA presence in Brussels, and for local groups this is a useful metric to measure success in one sense i.e. as an easy-to-generate quantification of the size of our group at any one time (as well as the historical trend) as well as for the movement as a whole.
It could also be combined with other metrics e.g.
with impact (e.g. GiveWell donations, 80k Hours career changes) to assess communications effectiveness.
with time (of the local EA team) and cost invested to achieve to assess operational effectiveness.
If compared across local groups, the metrics would highlight local success stories and potentially where groups might need help.