Yes! Unreleased songs are totally fine. It's a good way to line up coverage ahead of your release date.
If someone wants to share your song, we tell them that they have to wait until it gets released. When that time comes, we send you a reminder to update all your links, and we also notify the curator that your song is ready to share.
Curators specify their release-date preferences. For example, someone might say "I only want to receive unreleased songs" and someone else might say "Please only send me already-released songs." This means that you might see a different group of available curators to submit to once you've reached release date, so it doesn't hurt to revisit then and see if there's anyone good that you missed.
To submit a pre-released song, you can use an MP3 upload, private SoundCloud or an unlisted YouTube video. When uploading your song, you'll have an option to indicate the release date for your song, which will be displayed to the bloggers so that they know not to share it yet. Once your song does get released, adding the extra streaming links are easy. Simply navigate to your song (you should be able to find it on the homepage) and look for the "Additional sources" button to add your links.
Note that for private SoundCloud tracks, you'll need to include a secret key at the end. Details here.