Using latest version of Jetpack, latest WordPress Multisite
Regardless of whether I actually have mbstring installed, I find it interesting that when saving the settings for Sharing, whether I get an error message is inconsistent.
On the main site: no errors
On a subsite: Warning! Multibyte support missing!
This plugin will work without it, but multibyte support is used if available. You may see minor problems with Tweets and other sharing services.