Its one of only three games I play on Facebook anymore (primarily because the second hasn't been updated in a year and the third is being shut down on Dec. 12)and I must say it's a fun game. If you've never heard of it or never played it, it's a war game. Aside from resources, monsters and defense, there's not much to it.
If you're not someone who likes to play on Facebook, there's a version on Kongregate that's a bit behind Facebook on updates, and there's another on Hi5.com, which is now 12 months behind.
Sorry I forgot to mention that I am a decently high poster on the Kixeye Forums as well, so if you ever need any help with Backyard Monsters, just go there, and there's a pretty big group of people who can help you.