Very interesting and useful. But you can get backlinks from forums also. My site is built in Joomla so I went and searched for Joomla forums, mostly I found nacional forums from countries like are Germany, Italy, Spain, France…and those forums are very active. I checked if I can saw as unregistered user other peoples profiles and signatures, and mostly I can, so i add my homepage and signature with three links on those forums. And you do not need to write on forums to get backlinks.
