I think one of the reasons these sites can be particularly problematic in this regard is that people tend to join up with no intention of having affairs. So they are connected to their friends, wives and husbands much as they are in the real, offline world. But eventually they get embroiled romantically with someone within this network - or closely connected to it - so it's just much more likely that others within it will find out. The betrayed spouse inevitably gets the news and all hell breaks loose!
If people join dating sites they are going outside of their traditional social circles with the intention of finding sex, love or romance. To put it bluntly these tend to be "fresh fields" for the dating site member. They are contacting people they probably would never meet in any other way.
So there's probably just as much (or more) infidelity occurring via dating sites. It's just that the protagonists are less likely to get found out.