Justin Bieber was discovered through YouTube. And he helped contribute to Carly Ray Jepsen's career because he saw one of her songs on YouTube, covered it, and helped it go viral. The Weeknd used YouTube at the start of his career as well.
I don't think this phenomenon is limited to one form of media. "The Martian" by Andy Weir was originally published on his website one chapter at a time. The internet has, in general, lowered the bar to publishing any kind of work significantly - be it games on Steam, songs on YouTube, or ebooks on Amazon. There are now new avenues into the wider industry, but the big players are still the big players.
I don't think this phenomenon is limited to one form of media. "The Martian" by Andy Weir was originally published on his website one chapter at a time. The internet has, in general, lowered the bar to publishing any kind of work significantly - be it games on Steam, songs on YouTube, or ebooks on Amazon. There are now new avenues into the wider industry, but the big players are still the big players.