Some of the social media discussed on this site:
Commercial social media sites all either operate on ads (capturing your attention, profiling your interests, showing personalised ads, i.e. selling your data to advertisers), or on venture capital (investors are paying the bills in the hopes of selling it later to the public or to a big company). Future profits are promised via ads.
Edward Vielmetti then added another point:
Edward then wrote it all up in a blog post.