100G has been around for a while and not that common in usage, other than extreme cases. 10g and 40G are still the most widely used ports. Considering that most switches and routers have a minimum 10-20 ports (10g-40g), that's already a stupendous amount of BW - and on routers, you can always add more cards. 100g routers/switches are very expensive.

Unless you are pushing google/facebook/Comcast level traffic - there are very few use cases. Apparently, Google/Facebook uses their own network hardware.