Can someone steal your poem/song your write and post on the internet?

In Australia, any work that you produce - any poem or song that you write - is automatically granted copyright and immediately becomes your product. And that is all well and good if you can prove 'he said, she said' arguments surrounding its authenticity. So it is always best to ensure that you do all that you can to remain sole owner of your product; which may not be the case when using particular sites. Some sites have provisions that your work becomes theirs, to do whatever they want with. Alternatively, you may retain ownership but be prohibited from posting your poem or song on other sites. So it is always best to check the terms and conditions of a site, and to read and study them thoroughly.

Of course they can steal your poem online. It's so easy to do it. You have to have some kind of rights for your works. Try googling "creative commons" license.

Absolutely, yes! I write for Dooyoo and Ciao, and notice a lot of plagiarism on there where people have 'stolen' material from elsewhere on the internet, claiming it to be their own. If you get your work copyrighted, you can legally pursue anybody who's stolen/copied your work online. I'm not 100% sure, but I believe copyright laws apply the same online as offline.