Not necessarily. It can be all sorts of marketplaces, and some are not even marketplaces.
B2B: The original Alibaba, connecting Chinese manufacturers with Western importers.
B2C: Amazon 3rd party sellers, eBay, Taobao
C2C: Facebook, WeChat, Instagram
The best example of non-marketplace companies that have strong network effect is Google. The more people use Google to search, the better its algorithm becomes, and so more people search on it. In the PC days Windows has extremely strong network effect. Third party devs have to write on Windows because that’s where the users are, so users use Windows because that’s where the apps are.
This very good article argues self-driving cars will become another network effect. We don’t know yet how strong it will be.