Fair point. Photos definitely are (IMHO) more engaging than plaintext or even Google Maps of where someone is. But this post compared photo posts on either platform, so to be fair, the medium was the same -- but you're still right in that Foursquare doesn't have as much of a focus on photos as Path does.

Really interesting counter-question you posed. It appears to then be a decision about Path's conviction to how things SHOULD be designed vs. immediate conversion lifts (and how significant those lifts were/what future engagement was like/other conversion and engagement metrics).

For example, I imagine Facebook has tested designs that increased user engagement but didn't fit with a bigger picture strategy or violated some core tenets. They probably scrapped it and stuck to their conviction in a lot of cases when there were alluring potential upticks in conversions with alternate designs.