Making a plugin or theme work on multisite with shared global database tables

Use $wpdb->base_prefix everywhere instead of $wpdb->prefix

$wpdb->prefix and $wpdb->base_prefix
Shared database tables or individual tables per individual sub-site

Single Site

$wpdb->prefix and $wpdb->base_prefix are identical on a standard WordPress install

Multisite Subsite:

$wpdb->prefix – if you want seperate tables that belong to individual sub sites.
$wpdb->base_prefix – if you want table shared by all the sub sites (site 1, site 2, etc) (Multisite global).