I would use twitter for my own professional development as well as with my students. Ideas for these uses can be found in the #langchat article and the and Using Microblogging Platforms for Educational Purposes.
I would use #langchat to chat with other Spanish (or even possibly other language) teachers. We could share teaching tips or even new vocal words learned along the way. I could use this tool to develop my own PLN. Through these meaningful discussions about language, culture, and instructional methods, I could enrich my own teaching and at the same time help others to enrich theirs. Tools like these chats help teachers to truly become lifelong learners.
"Using Microblogging Platforms for Educational Purposes" states that microblogging is a great way to get shy students to speak up. Although I would eventually have to evaluate my students on oral proficiency; practicing their written Spanish through Microblogging may be a start. I would also use Twitticate and have my class create their own Spanish story. Finally, I would use the project monitoring tool to guide my students through long term projects such as essays.