I'm skeptical of explicit metrics like "number of GWWC pledge signers", "money moved", etc. Any metrics that get proposed will be imperfect and may fall prey to Goodhart's law.
To me, careful analysis and thoughtful discussion are the most important aspects of EA. Good intentions are not enough. (After you read the previous article, imagine if an earlier EA movement had focused on "money moved to Africa" as its success metric.)
The default case is for humans to act altruistically in order to look good, not do good. It's very important for us to resist the pull of this attractor for as long as possible.
Some possible criteria:
I like this list. We could improve on it by establishing a hierarchy of metrics.
1st Tier: more quantifiable and objective metrics which are also most strongly tied or correlated with direct impact.
2nd Tier: quantifiable metrics which aren't directly tied to increased impact, but are strongly expected to lead to increased impact. In this tier I include memberships which are expected to lead to more donations, and to overcome constraints on talent and human capital.
3rd Tier: metrics which are less direct, more subjective, less quantifiable, and are more about awareness than exactly expected impact.
I think it's possible for one metric to jump from one tier to the next in terms of how much confidence we put on it. This can happen under dramatic circumstances. For example, "media coverage" or "positive media coverage" would be something we would have much confidence in as impactful if effective altruism gets a cover story on, e.g., TIME magazine.