You are wrong regarding Windows phone doesn't, I'm using a windows phone myself with Twitter, Facebook and Instagram app. And yes they are from the social media themselves. With Instagram I can upload images and video.
social networking won't stop the web based, they can't otherwise they loss profits from advertisement and other mislays. Yes allot more people like myself are doing though phone since it's easier
I think Pinterest, and some smaller social networks are no longer making their app available on Windows Phone. But, most of the big social networks like Facebook, Twitter, and Instagram have a Windows Phone app.
Although, Google Plus, and YouTube are not available on Windows Phone.
I agree, social networking apps won't stop their websites because they will lose profits from ads, sponsorship, and virtual cash for games like FarmVille, and Candy Crush Saga on Facebook website games where there are some gamers who spend many dollars on virtual cash and items when they play these desktop website games on Facebook.
I think there will always be a large group of people like the unemployed, disabled, and retired people who enjoy using a web browser to go on Facebook, Twitter, and Instagram because they are at home a lot, so they can easily turn on their desktop computer, and spend many hours on social networks to see what celebrities, friends and family are up to, and to be less bored by playing a farming, sims, and virtual pet game online. There are also many users who stream free music and video which someone uploaded to a social site for fans to listen to.
Plus, a lot of people tend to eat snacks while they browse the internet at home, and a desktop PC or smart TV is one of the best ways to browse the web because the screen is bigger, and you don't need to hold your device.