I use it to stay in touch with my sister in New York, my cousin and Texas, and my mom and Grandma.
What about e-mail? Well, I know that my sister was sick over the weekend and her husband went out in the snow to get her soup. I know that they bought some little snow-slippers for her dog. I know that my cousin started his new job on Thursday and was very excited..
None of those are things they would have e-mailed me. In fact, getting all that by mail would be a bit spam-ish, to be honest.
But I'm glad that I have a place to check in on them and see how things are going. Likewise, they have a place to check in on photos of my daughter and see what she's doing every day. I'm certainly not going to e-mail them photos of Samantha every single day, but if the want to see some on any given day, they can.
It's a good "check if you want to but ignore if you don't want to" kind of system. E-mail is very in-your-face and demands an answer. Twitter is more passive. It's more like "in case you want to know"
rather than "here! respond to me!"
like e-mail is.