Your Worst Enemy Is Yourself
avc.com
avc.com
The only "competitor" you should worry about is the Back Button.*
*http://www.paulgraham.com/startuplessons.html, http://mixergy.com/paul-graham-design/
The concept of flow is often mentioned here on HN and letting competitors destroy flow is an easy trap to fall into. As others have mentioned, the question remains how much time one should allocate toward observing and reacting to competitors. While it is undoubtedly above 0, one has to take into account the opportunity cost involved as well as the energy wasted on switching gears, which can be quite significant and is often overlooked.
(I personally believe Foursquare should just keep doing what they are doing but let's discuss this anyway)
My sneaking suspicion is that for many bits of cheap, generic business advice, there is some equal and opposite bit of advice.
Edit: anyone remember this one from last week?
http://steveblank.com/2010/11/01/no-business-plan-survives-f...
I am not suggesting that you should put your head in the sand. It is critical to know what is going on in your market. But it is equally critical to have a strategy that makes sense in the context of what is going on and execute it with purpose and pace. If you spend too much time looking over your shoulder, you will not execute well. I've seen it again and again.
Just remember, you can spend a lifetime finding the optimal path, but you can only make a step now.