Acquirer is paying for the community, not the easy to replicate tech. There were lots of image sharing apps, but only one Instagram.
The first are captive, the second only allowed to post once per year, and the third is ideally a very temporary situation.
It's hardly comparable to a social network, or even other review sites (which can cater for enthusiasts)
It's not comparable to a social network because it is a social network: people and relationships and they're nodes and vertices on a social graph.