Why did a couple of my favorites disappear? I wanted to see them and they were gone :/

Two possibilities: it's a bug, or the content was removed from the site.

It's pretty easy to tell which is which: if you can find the content again - it's still on Fanpop - then it's a bug and you should report it (use the "contact us" link at the bottom of the page. For more detail about reporting bugs, refer to link). If you can't find the content again, then it's probably been removed from the site because the content was either broken or inappropriate.

There is a site called link that will track your youtube favorites for you. If you enter your Youtube username, it will save all your favorites, and any time you go back it will let you know if any are missing from last time and offer alternatives.