It's amazing how far Flipboard has come ever since it launched on the iPad. It was just a very simple news/rss reader which transformed everything into a magazine-like format. It was a pretty simple idea - it was executed magnificently. That was what made it so successful, it's execution. The developers made sure it worked properly top to bottom on every release - they knew that high quality work and design was required here to achieve a high quality product.
It was exclusively on the iPad for over a year, this made the demand for it skyrocket. Users saw how beautiful it looked on the iPad and wanted it on other devices. Then near the end of 2011 to the current day, Flipboard's development has shot through the roof. It first launched an official iPhone version of the app in December 2011, added a whole new bunch of features, social media integration and much more, then launched on Android in May this year.
The popularity of it is huge and the statement of 20 million users says it all really.
I still find that the app only works well on large screens like the iPad. On Android and iPhone I'd rather stick to FLUD.