Ask HN: What new, worthwhile ideas were outcompeted?

I was just reviewing a bit on Peter Thiel's idea of secrets[1], and how marginally simple secrets have to grow really fast to make sure competitors can't catch up.

I was questioning whether or not this was true. Does anyone know of startups or ideas that started out this way, and then were outcompeted by a later joining competitor? I don't mean something like YC's Kiko (a social calendar which left the space when Google entered). Something that had traction and was overtaken.

Thanks!

[1] http://blakemasters.com/post/22866240816/peter-thiels-cs183-startup-class-11-notes