There are many differences between Facebook and Twitter. One major
difference is, on Facebook people need to ask for permission to see your content. Twitter, while there are security levels to protect your content from public viewing, generally, anyone can see your information.
I say Facebook is for the people you know; Twitter is for the people you *want* to know. For me, there is no doubt, Twitter was life changing. I've been able to chat online with people I never thought I'd have access to. (Randomly one night, I launched into a conversation with Moosewood Cookbook author Mollie Katzen. Did you know those books really are written in her own handwriting? Not a font, based on her handwriting, as someone else suggested.)
My base of "followers" on Twitter is large enough that I use it often as a resource. Where do I find gigante beans? What's the best scale for bakers? That kind of thing.
I'd say people use Twitter in many different ways. Some for business, others for self-promotional purposes, and others for social reasons.
Facebook, to me, is more personal because I know most everyone who has access to my content. I use it to link articles I find interesting, note things that I'm up to ("I'm cooking for The Family this week. Here's what I made...). People with kids upload photos, which is really nice. I have friends in Arizona who I haven't seen in nearly 10 years but I see regular updates of their kid's photos. I love it.
It took me a while to grasp both Twitter and Facebook. I like them both, for different reasons.