I have won free products from Bob's Red Mill just for tweeting about a product of their I like or disliked. You can ask questions and re-tweet other people's posts to get the most readers out of it. There are also celebrities, news, and regular everyday people who use twitter everyday. And its FREE!
I use twitter to let the world know (or at least my followers) when the blog is updated and to keep up with new products coming out by companies like Bob's Red Mill, Kinnikinnick and Ians. I also follow other bloggers who are gluten free bloggers, they tend to post links to recipes and contests for free gf products on their blogs. To find the companies use the search feature and they usually come up right away.
Twitter is very easy to use. If you want to follow me I am @dispatchergurl on Twitter. Happy Tweeting.