If you post something on your blog and you want it to be tweeted to your twitter account or facebook account,
just does that. Super cool thing is they don't ask you your twitter user name and password and bank info, They use oAuth, it takes you to twitter site, you authorize twitterfeed to update your twitter feed, Bingo, You have it.
If you want your posts to be written on your facebook wall, You can do that with twitterfeed too. But I have twitter synced with my facebook already, so now having twitterfeed to tweet my posts to my twitter account, I am hoping my post will be broadcasted to twitter and twitter will write it to my facebook wall.