Facebook popularity is begin after Apps and then it provide awesome features like wall feed through this the user can be share each and every information about him/her self, friends and others also. And they like, share, comment their updates. So this feature is unique on that time so its boom on the market at the particular time being and become popular.
Thats the good feature of Facebook. Its a best medium social bookmarking. User can use the like nad share options and post articles, scrap etc.
Various kinds are made popular on facebook. Many people shared his friends information, comments, videos etc. Also I agree with your post hardly. I think these main cause faceebook made popular.