1. Virality - Highly viral product that got shared enough to the point that almost everyone knows about it (in current target demographic). Throw up that Mission Accomplished banner.
2. Engagement: Falls off here. Most early adopters got bored, and it's just now catching on with what I would call the "early majority." This would be fine except they're running through the same cycle of being bored by checkins. Reviews/Radar seems like an attempt to engage their users beyond check-ins, which probably seems more confusing than anything else for the typical early majority user. I think the problem here is that they've lost the "core user group" (aka that bored early adopter) you're talking about...the type of people that would spend time curating lists, putting up great content, and getting a kick out of people checking in to their recommendations.
3. Monetization: Nobody seems to care about this anymore, but with their level of funding/valuation, it's probably becoming increasingly difficult to tell them that they're just a few pivots away from making money. However, until they have a clear idea of how people will engage with their product, I'm not so sure they'll know how to monetize. It seems like 4square is pushing towards some sort of CRM system for local merchants, but I don't think I'm smart enough to know the right answer to this one.