So you are retracting your comment of earlier
“If someone is happy in their life and marriage, they are not looking elsewhere for satisfaction”
Ok. Fine, that is where I was getting. That your comment is not even close to the rule.
The fact is someone can stray even if they have a perfectly happy marriage. And the wise thing to do is to avoid things that can cause straying.
I am not saying facebook will always cause straying or even does so a significant amount of the time, however one can never be sure (not my words, its the gemara’s words) and it is therefore samrt to be cautious.