If you use Facebook and/or Twitter, Siri offers a bunch of helpful options. You can say “Post to Facebook I love reading Lex and Dan’s stuff at Macworld.com,” or “Write on my Wall I love Siri,” or even, “Post I’m an over-sharer to Facebook.” With Twitter, you can say “Tweet I’m eating breakfast” or “Post I’m still eating breakfast to Twitter,” along with other, similar variations.
This is an interesting article presenting why businesses need Twitter.
Why You Need Twitter In Your Business
A quick and plain English intro the micro-blogging service Twitter.