I think it depends on your goals. For example, "top of the funnel" type of efforts can drive big numbers. If the goal is to reach a broad audience, gain "mindshare" and recognition, than measuring hard stats on things like traffic and social media engagement are useful. However, sometimes developer relations focuses on existing customers. A post about how to do X with our product Y fills a real need customers have, but won't gain much traction in terms of traffic and social media engagement.

This isn't specific to writing. Speaking has much the same issue.

Once you add in, as Todd said, having to play to the specific "evangelist's" strengths, I think it becomes tough to come up with number goals. You end up having to rely on qualitative versus quantitative data. Unfortunately, this sort of data doesn't always translate well up the chain, so DevRel always tends to face the "I'm not exactly sure what they do for us" conundrum - we do a ton, it's just difficult to measure the impact.