The feeling you experience when, on reconnecting with an old friend/lover on the net (myspace/facebook/classmates/etc), you discover that they appear to have a better career, better sex life, nicer family or more money than you.
"Remember that girl Trish I went out with at school? I got in touch via facebook. Turns out she got into modelling and married a millionaire. I had a serious case of nealousy."