3. Parents who do not monitor what their adolescent children are doing and posting on those sites.
P. S. Perhaps qb should have been more specific, judging by the first comment below. In particular, I loathe the nefarious and socially irresponsible uses to which Facebook and MySpace are routinely and pervasively put. I do not wish to outlaw them and would certainly object to such a ham-fisted, sub-American remedy; I simply despise the platforms and their cyber-kin in all but their most innocent deployments.
There is a story behind this, a story which will remain untold, a story that will ring true to parents who are swimming upstream to keep their beloved adolescent children as innocent and as thoroughly protected as they can from the ubiquitous evil that emerges during the pubescent years. I am truly shocked.