Its usage has increased, but so has the usage of other languages. Its drop in popularity is relative to its share of all users.
If Alice has two apples and Bob has three apples, Bob has 60% of the apples. But if Alice gets four more apples and Bob only gets one, Bob has only 40% of the apples, despite having more than he did before.
It's the same with TIOBE's popularity index.
Given how much lua is used as an embedded language in Games, Databases, Servers, etc. I feel I've only seen it's usage increase.
Go I can see as "appearing more popular then it is due to HN" but even then I feel like clis/networking tools have been using Go more and more.