The real dilemma is this: if Path A/B tested their current design against one similar to that of Foursquare, and the Foursquare design had better conversions, then which one should they choose?
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.
Path is inherently about the photo first, I'm never clicking through to Path to figure out where the person is, only to look at the photo.
Arguably if 4sq. started including photos sooner, or had a less boring UX on those UGC landing pages in the first place even if they lacked photos for a while, they could have counteracted this sort of apathy towards their links and avoided, or altered, the mental filter you mentioned.