Features define the market.

Spray paint manufacturers aren’t in pen market even if you can “recorded text and images in a durable fashion with them.” Lighting a couch on fire sure produces a lot of lights but couch manufactures are not part of the home lighting market.

EDIT: But let’s ignore that. Sure, YouTube has roughly the share of social networking market as the number of people posting vacation photos on it. That might just add up to 0.0001% market share.

If you define the market as [the previous definition] + [photos], Facebook still has significant competitors.

I can’t think of any definition that would only apply to Facebook that isn’t so narrow as to be unprecedented as a definition of an entire market.

Bell was “telephone service”. Standard oil was “oil”. What’s a comparable definition for the market that Facebook is in?

I would suggest the overlap between Facebook and G+ is a reasonable definition for broad real life social network for communicating with real world fiends, family, and social activities. LinkedIn, Patron, Reddit etc have related features but are designed for different kinds of relationships and types of communication.

Arguably Instagram in April, 2012 was a reasonable pens vs pencil division where the courts could have included or excluded them quite reasonably. Though allowing Facebook to buy them suggest a vary narrow definition at the time.

By that vague definition, any app/site with UGC is a personal social network which is not a very useful definition.

A definition seems hard to come by. I've been informed here in the past that Hacker News is a social network too. It's not implausible to me, since it's a site where you can comment about stuff, just like Facebook.