I think I would start more simply with something along the lines of
"want all of our close family and friends to know that J and I are mutually choosing a polyamorous lifestyle for ourselves. If any of you would like more detailed info, we would be happy to elaborate."
Give people some time to digest that (if they r on fb, they likely can do a google search on what polyamorous means). Those who couldnt care less wont mention it, those with interest, fear or concern will.
Then after that "intro to coming out" just start posting your happy moments/thoughts or whatever regarding both partners.
But-thats just me. I am out to everyone in my life. Thats roughly how We came out and it went very smoothly all of the way around.
"Love As Thou Wilt"