Line gives you free voice calls (like Skype or Facebook’s new overhauled app). Then there’s basic messaging, but Line is a bit goofier with sillier emojis and stickers. There are teddy bears juggling eggplants, bunnies with flames of anger in their eyes, and a shy balding man surrounded by little sparkles and flowers. (Yes.)
Japanese trends do sometime take off here in the US. 100M is quite a large set of users. Facebook has about 1000M users.