(Besides what has already been mentioned regarding some AM3 mobos support FX chips with a BIOS flash, and that AM3+ offers power delivery improvements)
Intel does a socket refresh (almost) every time they introduce a new processor family. Whis is it such a big deal when AMD does the same. I can see this as both good and bad:
Good) New sockets show that (hopefully) significant architectural changes and/or feature additions have taken place to the CPU, Chipset, or both. I feel like you can only progress so far by keeping with the same socket. I think Intel updates sockets too often and AMD could use more frequent socket refreshes sometimes. But I don't think either company should be required to stick to a given number of CPU generations on a particular socket. If something needs to be changed it should.
Bad) As touched on in the good, we consumers are then stuck with the burden of having to buy a new motherboard along with that new processor. Nobody likes that. That's also why I tend to spend less on motherboards for my Intel systems. I don't mind paying a bit more for some luxuries on an AMD board that may get me through a processor upgrade 2 generations later. That being said, a lot can change in 3 years. (loop back to "good" section)
FM1 was designed to accomodate the IGP in Llano processors. It was never meant to house FX CPUs. The faint rumors for FM1 compatability with Trinity were only floating around shortly after the Llano launch. But I don't recall there ever being an official guarantee. AFAIK, it's been known for quite some time that Trinity would not be compatible with FM1. That being said, FM2 is said to be compatible with Richland APUs next year and maybe even one more beyond that. AM3+ supports Bulldozer, Piledriver, and purportedly Steamroller FX CPUs as well.
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Hand-Built Wood Case