It seems to me, if Instagram could really only chose one metric to track it would be an app visit, not a photo upload. Instagram must have users who are primarily content creators and another larger group (probably a superset) that are content consumers. Not focusing on the actions of the content consumers would seem really bad for business since what early-stage Instagram's investors care about is eyeballs not photographs and what late-stage Instagram's advertisers care about is eyeballs not photographs.

Really Instagram probably has a few OKMs (visits, uploads, hearts, comments, follows). All of those are important and should be tracked. In their documentation and educational videos, I've not seen Mixpanel focus on visits as a key metric. I'm not sure why.