Since music is inherently a form of self expression, and due to the sight, sound and motion of video, people loved the idea of identifying themselves with their favorite music video to show all their friends and strangers their personality.
It fit really well with the mishmosh profiles on MySpace and grew incredibly fast. Within 6 months we had 40 million videos being streamed daily! I eventually sold that business to a NYC based startup and became the co-owner.
But one of the things that we did to SEO the crap out of our site was to dynamically change the href anchor tags in our embed codes for all of the keywords we wanted to target for SEO purposes. There were like 500k embeds being copy / pasted daily, so you can imagine how rapid our SEO strategy was spreading...
If you're looking to virally create backlinks for your site to better SEO it... perhaps you should look towards viral embed widgets that still exist, and tell them you'll pay them to link to you for the keywords you're trying to SEO.
i'm not sure how google reacts to this today, but back in 2004, they placed us at the top for the results for all of the keywords we targeted.