How do sites like spotify and facebook stop similar sites existing?

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Spotify is unique because it is the only free one of its kind. The paid version is better than its competitors (yes there are some similar ones) because it load the songs fast, looks good, easy to use etc.

Facebook stays ahead of its competitors because it is always bringing out new features, there are loads of apps, the majority of people use it so its easy to find people you know etc.
I remember a few years ago bairly anyone at my school used facebook and everyone was on bebo. But things change and everyone has moved to facebook. The live chat is a great feature aswell that isnt seen on many social networking sites.