Most big sites by default use ‘nofollow’ tags therefore not trusting external links nor users that post those links there. This means that they don’t give any credit (in the form of PageRank) to those links and are essentially considering them spam (as that was ‘officially’ the purpose of ‘nofollow’ tag introduction – actually messing with the blogging community search engine rankings was the real purpose). This results in their pages appearing higher in search engine results and your pages appearing lower.