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If you paginate an article then you can also use the rel=prev and rel=next to help Google crawl the pages in an article. I would be careful with UGC.  You may have someone with a list of 100 songs, but then they only comment on 20 of them.  Is it really worth showing all 100?  Seems like you would be ripe with duplicate content with all the repeated titles of songs.  Why not show the top songs with comments or maybe most played with comments?   Put the other 80 on a next page and then canonical link the additional content to the main page. I would worry less about length and more about quality and originality. Cheers!

do you have enough unique content for 8 million songs pages? http://www.seroundtable.com/google-too-many-pages-16664.html

It's not going to get them deindexed and it's unlikely it could be the root cause why they wouldn't rank high. There's honestly not much content on each page, nor it is easy to tell what keywords you are targeting since they kind of cannibalize each other, and that makes it harder to rank well.