Hey Vincent,
To answer your questions…
1. Yes, all of our plugins (Custom Facebook Feed, Instagram Feed and Custom Twitter Feeds) work in a WordPress multisite environment/setup.
2. No, you do not need to purchase the extra licenses in a WordPress multisite environment/setup. You would install the plugin on the Admin Site and then Network activate the plugin so it becomes available to all the sub-sites below the main site. Not only is this a cost savings but it also makes license management, plugin installation, maintenance and updates easier as you only have to perform those actions on the plugin installed on the Admin site. The properties are then carried downward to the other sites due to the network installation.
Also, the plugin will allow each sub-site to maintain it’s own plugin options settings so you can customize it on each sub-site as desired.