I think it's just one of those companies whose worth is not obvious. I suck at figuring out how services monetize their data, but as I understand, their API is nothing to scoff at.
Maybe it's more of a data service than a social service now?
I made my comment based on the fact that they don't seem to be adding any new features. It still remains a manual check-in service, while there is a lot more you can do beyond that.
But reading your comment made me reconsider mine. After all, Twitter also are still the same as when they started (vs Facebook, who are constantly adding new features). So now I'm not sure anymore about my original comment. Should I delete it?
In the future, I shouldn't comment on such things before actually using them.
I'm not a user of FourSquare, but the comment struck me as far from a fact.