Those are by far the dominant social networks in the US. They all compete in terms of fighting for limited user time. WhatsApp is kinda sorta in that group, however it's not particularly popular in the US. Tumblr used to be, it might as well be dead now, it's going the way of MySpace (sold off a few times, passed around, until it's hardly valuable at all).
Snapchat competes primarily with Facebook and Instagram. At the edges there's some competition with sites like YouTube, Reddit, Imgur (and other image sharing networks).
Instagram competes with core Facebook, which is of course why they ate them. Also with Snapchat. To a lesser extent YouTube, Pinterest and misc image sites.
YouTube competes with Facebook and their goal to build a much larger user video platform. To a lesser extent they compete with Snapchat and Instagram, particularly on very short form user video.
Reddit competes with Twitter and some of the smaller niche social networks like Imgur. There's some limited competition between Reddit, Instagram and Pinterest.
LinkedIn competes with Facebook. Microsoft scored a nice protected fiefdom in LinkedIn. It suffers no serious competitive threat other than incompetence.
When you examine the competitive landscape, it's very clear that Facebook has no direct competitor to their core platform. And further, Instagram has far overwhelmed its competition. Facebook core is Facebook's Google search, Instagram is Facebook's YouTube, all are monopolies at what they do. The scope of Facebook core + Instagram + WhatsApp is breathtaking, it must be something like 2.6 to 3 billion monthly actives now when you include everyone across all three major platforms.
The most interesting social network among the group, is Pinterest. If they don't screw things up, they'll have a far better business than Twitter over time. It's a dramatically more mainstream product with direct, obvious, superior monetization potential. None of the other social networks can compete well with what they do (not without contorting themselves into a pretzel and risking their existing userbase and business).
That will turn around quick if Pornhub actually succeeds in buying it.
What do you consider to be their obvious superior monetization potential? And is their anything else about Pinterest that makes you find it so interesting?