fontawesome does not show symbols - xelatex, lualatex

You try to use the fontawesome package, but this package is only compatible with Font Awesome 4. It will lead to problems with Font Awesome 5, especially since there is no longer a single font including all of Font Awesome.

Instead you can use the fontawesome5 package:

\documentclass{article}
\usepackage{fontawesome5}

\begin{document}
Twitter: \faTwitter
LinkedIn: \faLinkedin
Envelope: \faEnvelope
\end{document}   

Disclaimer: I am the maintainer of fontawesome5.

To be able to use Font Awesome 5 with [Xe/Lua]LaTeX, the easiest solution is probably to install the font on your system and access it through the package commands from fontawesome5 for the respective symbols (like you show in your MWE). There are other possibilities, if you don't want to install them system wide, but this is for another question.