I have this JSON:
{ \"client_id\": \"26075235\", \"client_version\": \"1.0.0\", \"event\": \"app.uninstall\", \"timestamp\": 1478741247,
SelectToken("data[0].user_id") doesn't work because there isn't an array in your JSON. You should use SelectToken("data.user_id") instead.
SelectToken("data[0].user_id")
SelectToken("data.user_id")
Fiddle: https://dotnetfiddle.net/K0X4ht