[rome]

Simply copying someone's idea isn't always enough to being successful with it. Google+, a social network clone, still can't compete with Facebook. All of the Instagram clones don't have anywhere near the amount of users that Instagram has. Execution of an idea is what counts.

[stevenwei]

Both Facebook and Instagram have network effects that making cloning less effective (not to mention they're both free).

Utility apps sold via the App Store are indeed much more easily (and profitably) cloned.