It's not an attractive trait, but most of us secretly (or not so secretly) yearn to be doing rather much better than the ex after a break-up. Whether it's the first one to find someone new, or to be clearly financially better off, or even to look younger / slimmer / fitter, it's often somewhat of a race to out-do each other (consciously or subconsciously).
Obviously, in order to know if you're the one who's 'winning' you have to somehow find out what the ex is up to. Sometimes this can be in the form of interrogating mutual friends, although this is a risky option which can end up alienating you from people (opening a whole new can of worms when word gets back to the person you've been distancing yourself from emotionally & physically for the past few months). However, with the world-wide-web at your fingertips it can be as easy as typing a name into google... and here lies the thin end of the wedge.
At a time when you need to be moving forward with your new life, and leaving the past behind, DON'T be tempted to waste even a minute of your day spying on what he/she has been up to since you split up. If you find out they're doing better then you'll feel hideous, and if you find out you're the one who's currently on top then you'll want to keep checking back just to make sure you remain on top. Resist the temptation. You wasted enough days / weeks / months / years when you were with them... don't waste another minute of your life obsessing over someone who's not going to be a part of your future.