In summary, in common terms, mandatory deposits aside (which as far as i see it, do not apply to Songstuff directly):
No matter where your music is made available, or where you are based, posting your songs on Songstuff and other forums, blogs etc is a useful chain of evidence when it comes to proof of authorship and / or proof of ownership. This is useful if you are ever in the unfortunate position of having to prove your ownership or authorship.
If you are based in the UK or in many other countries posting on Songstuff IS publication of your work. This is NOT the case in the USA, or at least it is not clear cut as to whether posting on Songstuff is publication. (note: posting by Songstuff staff, for example as part of an article on the main site IS definitely publication, it is the self-publication of blogs and forum posts that is in question).
In the UK and many countries there is no need for registration of copyright. Copyright is automatically established on publication.
In the USA and some other countries (Check your country's intellectual property laws) you need to expressly register your songs to establish a copyright claim. This is done via the US Copyright Office. It costs $35 per song, or per collection. US citizens are supposed to do this.
For non-USA citizens, this is mainly where the debate above has occured. There is an agreement that registration should occur in order to pursue any claim through a US court (for any infringement that happens within the US jurisdiction). Where the debate seems to be is on whether you need to register your songs up front (prior to any infringement) or just prior to any claim.
My personal opinion on reading the information available and after discussing it here and elsewhere is that if you make your music available in the USA, to the US market, then you would be wise to register your music with the US copyright office. I am not a lawyer, it is my personal opinion. I am sure your local songwriter's society will be able to advise you appropriately.