When are market slows or begins to meet it’s natural market size (i.e. when supply meets demand), it’s natural for people to look for potential in related / competing categories.
It's also a function of people investing and creating a body of work and finding new and creative ways to reuse and repurpose this content and introduce their ideas to a new audience.
Combing, removing, inverting & reversing are all basic building block for the innovator.
Some people / brands commit early to a single platform. That could be a Social Network (life Facebook, G+, Linkedin or Twitter) or a Content Network.
When you have a large following it makes sense to invest in that platform
For them it’s natural for them to want to leverage and gain utility from their following, but not to miss out and the attractive properties of other forms of media.
Slideshare, YouTube, Lists etc are all destination search sites. People go there to find content. Creating compelling content on these platforms ensures you get higher search ranking (a ranking that is independent of Google).
Given to use these platforms you need to create content in a specific format, it makes sense that people will experiment with repurposing content, ideas and formats from other platforms.