I think divorce happens as much in the poly community as much as it does anywhere else. While being poly I've divorced and remarried, and two of my partners have separated/divorced from their partners. None of those were to leave one person for another but for the other reasons typical for divorce - cheating, long term changes leading to incompatibility, the dynamics not working anymore, etc. I've seen posts about, and have known people who have had marriages end when one of them wants to be either mono or poly with another partner though - I think it's just about how healthy your relationships are in the first place, how accurately you are reading the health, compatibility and mental state of the partners and relationships you already are in. Those blindsiding surprises usually seem pretty expected in retrospect.
And Okcupid.com for dating too - If you want to be non monogamous, it's a good idea IMO to put that you are looking for nonmonogamous or poly relationships in your profile, seeking out non monogamy, polyamory, and open in keyword searches to find people who are already experienced in open relationships can save some headaches.
Happiness will never come to those who fail to appreciate what they already have.