Huh. There go all the equally invalid stats about how great social networks are! ;-D
Initially I thought it might be a split between people who consider the social media presence part of their professional "brand" (part of your job) and those who just like to gab. People with a "job" element to deal with obviously have to fret about keeping the worlds separate, compliance issues, too many contacts, etc.
But looking at the raw numbers, these are not huge friends lists -- I mean, my mom is pushing 160 friends and we are not exactly aggressive FB self-promoters. It's not our job face.