There are various ways for engaging students in writing online.
Blogs
Blogging can be a fun and useful tool in a blended course. Some may prefer it to the threaded discussions as it is simple to upload images and videos and everything is on one central page.
Tumblr is what is commonly referred to as a photo blog. It's easy to quickly post and share text, photos, quotes, links, music, and videos from your browser, phone, desktop, email or wherever you happen to be. You can customize everything, from colors to your theme's HTML.
Wikis are the most useful to facilitate collaborative writing or organize projects.
Wikipedia is the prime example of a gigantic collaborative wiki that millions of people write on.
Think carefully before you require any tool that is unfamiliar to students. Sometimes they resent having to learn a new technology if they don't really see the value. They also get confused when needing to use too many different tools in one class. It's best to avoid using a technology unless it is the perfect tool for the job.
Vlogging
Video + Blogging
Combines the concept of student feedback, but rather than writing, students respond with a unique video they create.This video can then be embedded within a blog or in their Learning ePortfolios or in a class Youtube Channel.
Think Visually
Twine is a free web platform that lets you organize your story graphically with a map that you can re-arrange as you work. Links automatically appear on the map as you add them to your passages, and passages with broken links are apparent at a glance. Rapidly switch between a published version of your story and the editable one as you work.