If 2014 was the explosion of Facebook as a mobile app marketing and user acquisition channel - 2015 is shaping up to be the year of alternative channels. Pinterest announced today that they are supporting app downloads, where users can install mobile apps directly from Pinterest with a new product called App Pins. 
When you come across an app Pin, tap Install to download the app right to your iPhone or iPad without ever leaving Pinterest (you’ll only see app Pins when you’re using the Pinterest app on your iPhone or iPad). - Oh, How Pinteresting!
This is a great addition for Pinterest users as
75% of Pinterest's traffic is from users on mobile devices. Part of what makes the Pinterest App Pins announcement so cool is that App Pins are not ad units. More app discovery and user feature than a monetization play by Pinterest. Along with this announcement - Apple has created an "App Store Board" where they feature various apps on fashion and recipes. 
Pinterest currently only supports app downloads for Apple/iOS, sorry Android users....
support direct mobile app installs?
The Largest Social Networks by Alexa Rank are:
Key Stats:
84% of its daily audience visits from a mobile device, and 69% of ad revenues coming from ads shown on mobile. Support Direct App Installs?:
YES - from Ads Only 
Key Stats:
80% of active users are on mobile. Support Direct App Installs?:
YES - via App Cards App Cards are essentially "rich" tweets that can be posted and/or promoted - but promoting is not required. 
Because the lifespan of a tweet is roughly 17 minutes - finding the right mix of sponsored tweets is probably required for any significant user acquisition efforts.
Key Stats:
65% of users access via mobile device. Support Direct App Installs?:
YES - via
Tumblr Sponsored Apps 
Key Stats:
50% of video views are from
mobile devices. Support Direct App Installs?: **
Yes - with "TrueView for mobile app promotion". ** 
LinkedIn - not yet. Will probably add this feature when more B2B, SMB and/or Enterprise companies adopt mobile for their target market, clients, partners or employees. Of the newer social media platforms - including Instagram, Vine, and SnapChat - none currently support direct app installs and the fit/need is not clear as these platforms continue to grow and evolve. Want More Data-Driven, Mobile App Marketing Content?
Ghostly happenings are among us... and in your app listing too? If you aren't leveraging the power of app seasonality to make relevant tweaks to your store listing you're leaving precious engagement and conversions on the table.
Developers on the iOS App Store should plan in advance of the upcoming Holiday Schedule to allow enough time for apps to get approved during the busy holidays.
App Store Optimization is an involved process that should be regularly revisited based on recent changes in trends. Iteration is one of the key drivers for success in ASO.