I have spent a lot of time analyzing both PewDiePie and MrBeast, the two most dominant creators on YouTube.
Besides the regular CTR and watch time optimizations that all YouTubers do, I have identified 4 pillars that PewDiePie and MrBeast use to recruit, retain and expand their follower base.
Here is a summary of what I learned.
PewDiePie engages a lot with his fans on YouTube. He mentions them their comments in his videos and takes recommendations into account, like which game he should play next. He also features the creations of his community in his video series LWIAY.
MrBeast goes even further and directly flies his subscribers to feature them in his videos. He spends a lot of money on his fans. In one of his latest videos, he gave one of his subscribers the choice between getting a Lamborghini or a house.

Another way to increase participation is to start a common project with your followers. Something cool that both PewDiePie and MrBeast did is that they raised millions of dollars for charity using their platform. MrBeast and his army even recently launched a big campaign to plant 20M trees, and received donations from the likes of Elon Musk.
Loyalty comes from trust, and trust comes from being authentic and delivering quality for a consistent amount of time. Every brand wants to maintain, and ideally grow the loyalty of their audience/customers. It's a success metric. Think about what differentiates Apple from other companies.
The biggest sign of loyalty I have seen on the web in the past few years was during the battle between PewDiePie and T-Series, an Indian media company, for the number 1 spot on YouTube. It became a worldwide phenomenon, and his army was there all the way to support him for months in this war.

Creating an enemy is actually an extremely powerful way to rally your troops, but not everyone can orchestrate such an epic event that lasts months. Especially given the fact that T-Series mostly ignored PewDiePie, so he created the biggest clash in the history of YouTube, and maybe even of the internet, by himself. That takes a lot of skills, and that takes a loyal fan-base. The genius of PewDiePie was to create a narrative of a gigantic war, and because he already used to call his subscribers his "Bro Army", this was a perfect fit. The result: he gained 30M subscribers in 8 months.
You see so many YouTubers asking you to like, comment and subscribe at the end of their videos. PewDiePie and MrBeast are no exceptions. However, the way they do it is what is interesting. They built a unique relationship with their fans and because they are entertainment channels, they often use humor to convince you to subscribe.
For example, in nearly every single video, MrBeast leaves an original pinned comment asking people to subscribe! These comments generate a lot of engagement.

Even though YouTube is their primary source of activity, both of them understood the importance of building multiple streams of traffic.
PewDiePie has an incredibly engaged community on Reddit. He has one of the most active subreddits, with more than 3M subscribers there. His fans create memes every day and he regularly reviews them in videos and gives them shoutouts. That's how he keeps the community alive. He empowers them to continue.

MrBeast takes a different approach. He engages his followers beyond YouTube by giving them ways to participate. He often creates random challenges, like this one on TikTok where he asked people to guess how many jelly beans there were in that jar. He got close to 1M comments.

On Twitter, he can do something like giving $10k to random people who retweet one of his tweets. People get obsessed with this sort of stuff. MrBeast is like a lottery, both financially and from an entertainment perspective.

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If you enjoyed this, maybe you will be interested in my YouTube channel.
I make analyses like this one in an exciting video format. I spend around 50h per video :)