When I think of 21st century learning, I think of things that I did not have in elementary school or even in college 15 year ago...where I didn't even have an email account.
I think the Wiki can be especially useful as a communication tool for ELL students. My students are at various stages in the language acquisition but I'm a strong believer in having them read and write as much as possible. I think this would be a great way for them to write in a non-threatening environment where they don't need to worry about being criticized. I know that they appreciate the use of technology because I often have them email me assignments, and they jump at that chance. This would be a new stage for them to be able to communicate with me and one another.