Claiming that going on Facebook is causing divorces is like claiming that heart attacks are caused by indigestion. There might be a connection (unhappy people with roving eyes look all over the place), but one doesnt cause the other.
Certainly, the fact that people with marital problems report issues with internet/social media doesn’t prove which is the cause and which is the effect. It makes a whole lot of sense that unhappy people look elsewhere, as you say, but the easy access makes it more likely, and there have certainly been plenty of cases where the cause was someone giving in to the temptation of the internet, causing the unhappiness.
It’s way too simple to blame the higher divorce rate on the internet/social media; there are many factors. There’s no question in my mind, though, that it hasn’t helped.