Simply put, you write in a different "voice" and, generally, talk about different topics. So while I posted an "endorsement" of trash can liners on Facebook this morning, I probably wouldn't tweet about trash cans - or garbage bags. FWIW I'm buying the special trash can liners from SimpleHuman.com. This socially media savvy company (they also have a blog) places a "Share via Facebook" link on the checkout page. Clever.
Now I am writing about this on my blog, but only as an illustration of how these platforms are different. And yes, the different audiences are key to what makes me change my message and voice.
On Facebook, I have enough family members and friends that I figure they won't be too surprised to learn that I have garbage in my house, I must dispose of it and that I've discovered a cool new garbage can and special liners for it.