I think it is probably related to self worth. You get so used to being stepped on all the time that you just give up on trying to avoid it and accept that it is just the lot you've been dealt in life...

My husband did not cheat on me via FB. He actually did not sign up on FB until 2 years after his second affair.. those women just happen to be on there too. I think I joined mostly because of the fact the he had joined and I wanted to keep an eye on him, in a way. Not as simple as that really, but more that I feel more comfortable if we have fewer individual relationships (his friends and my friends sort of thing). Up until he joined (which was mostly work reasons), I had avoided it. Despite my friends asking me to join, I had no desire to do so because of the fact that my family is all on there.