Some ways that your students can use Twitter for Project-based/Task-based language learning:
(1) Use the “reply” function to start or join conversations as a part of an opinion-based writing project
(2) Search for resources and trends to talk about in a class discussion
(3) Find and follow subject matter experts in order to research a class project
(4) Become a subject matter expert, and attract followers as a part of a long-term project
(5) Broadcast information, reports and resources to classmates on Twitter. Each week a different student could be responsible for posting info.