It seems to me, after quickly reading liquisoap documentation, that Faust is much more low-level: liquidsoap would connect different audio effects, whereas Faust can be used to write the audio effects, at the level of sample-by-sample processing.

I would suggest to actually look at the Faust website and you will see that they are completely different types of applications.

I found this reply helpful: https://news.ycombinator.com/item?id=28413883