It could potentially share the backend -- track only those people who are on the free version, but keep everyone on one network. Also, no. Social media existed

It could potentially share the backend -- track only those people who are on the free version, but keep everyone on one network. Also, no. Social media existed before the current big general-purpose networks. See Usenet, email lists, IRC, forums, etc. A single big network with no particular purpose or common interest provides different benefits, better in some ways and worse in others, but niche networks still have their own worth.