I think that notion is utterly ridiculous.
Facebook is simply a networking and communication tool.
I mean if you think about how many people "hook-up" after going to a bar, club, party, or some social event, Facebook doesn't really play any kind of role in those encounters except maybe providing a way for those individuals to keep in touch afterward.
Define "Occasions of Sin"
Facebook could very well be considered an occasion of sin - in that - it tempts the weak. Some people who would never think of contacting an old flame face to face or think of going to a bar suddenly have that means at their disposal and may even go through with it innocently enough - then find themselves hooked on it and suddenly reality is a blur.
The Devil is subtle in his temptations.
Ultimately the onus is on the individual not to sin - but likewise - the prudent dont place themselves in occasions of sin if they have a propensity to sin in that regard.
In my view - Facebook can indeed be an Occasion of Sin. Just like an alcoholic shouldnt go sit in a bar without an intension to drink or a smoker go out on a smoke break with fellow workers not intending to smoke. If humans were not weak - then the devil would not tempt them like he does.
Being naiive to the dangers of Facebook doesnt make it safe. Saving ones soul is the most important thing we do. Not typing on facebook.