With things like twitter, the ui is not the hard part. Things like moderation are the secret sauce. All the corner cases and support for devopsy stuff likely account for a lot. Routing to specific instances for celebrities and such.
I originally read it as it's easy to clone twitter.
My response is it's easy enough to build a micro-blogging platform/service. it's all the other shit like moderation, regulatory/legal compliance, making a profit, keeping advertisers happy, etc that's hard.
With things like twitter, the ui is not the hard part. Things like moderation are the secret sauce. All the corner cases and support for devopsy stuff likely account for a lot. Routing to specific instances for celebrities and such.