I had never really thought about UI as digital body language at lease consciously but it make perfect sense. As Marshall Mcluhan said "the medium is the message". Having a fashionable platform for a particular cohort is particularly important for service/product success and adoption. Back in the day geocities was pretty popular then friendster. It takes a lot of effort to keep your UI in shape make sure you work on it.
This UI has a special role: it is a social mediator. It’s standing in for the user and speaking for them; the UI becomes part of their digital body language. This is a huge amount of power and responsibility.
Source: Facebook and How UIs Twist Your Words | Medium| Chantal Jandard, Jan. 2016