bimalpoudel usually not illegal but it depends on the specifics of each app and its code.
All the clones do reinforce that it takes a lot more to build a successfull business then just the app.
Ted S that is not strictly true regarding second player.
Vimeo is very successful in the video space. As is Facebook which entered video long after Youtube. There is a strong opinion in the business community that second starter has the real advantage as they get to learn from the mistakes of the first guys and build a more profitable business.
As you say, facebook looked at what the first guys were doing wrong and improved it.
There is also a healthy trade in clones of successfull US businesses in other countries and languages around the world. Many of these dominate their market and the likes of Facebook, ebay, Groupon and others come in a poor second.