On Twitter, the Hashtag #comments4kids is used to share blog posts written by children so that others may find them and leave the child a comment. The comments give the blog post purpose, the children an audience and help to encourage children's writing.
In our blog post about Blog Books we offered a way for classes who wanted to blog, but for whatever reason were currently unable to, a way to start their blogging journey. This is another step that could be taken.
If children produce work and it's place online, but not in a blog, a Google Form could be set up to allow people who view the work to leave a comment. An example of this can be found here. Through the above link and QR Code, our children received some nice comments that, if needed can be moderated, from 'strangers' who kindly read and commented upon their work.
Again, as we said in that previous post about blogging, if you ant advice about blogging? We'd suggest contacting David Mitchell.