Trying to build out the next generation mobile platform? People never raise money for their current product.
Why do they need to become a mobile platform? The founders can charge $0.99 for an annual app subscription and sit back and have a great income. Instead $100M will evaporate trying to become another FB/Twitter, which doesn't even make sense.
People had literally this exact same sentiment about Facebook in 2008-2010. ("It's just a photo sharing site for kids...") And they also suggested adding a subscription service. Instagram sold before they got along to integrating ads (they had less than 20 employees), but that product was/is on a similar trajectory. There doesn't exist a single large social network that directly charges users, so I don't think you can just assume that's a valid strategy.
When you're on a wave like this, you ride it. They likely sold <15% of the company for that $100MM. Sure, the current product is silly. But it's unbelievably sticky for users and really fun. If you don't understand this, I bet haven't used it at length. It feels surprisingly like Twitter did early on. A new medium. A new protocol. A new way of interacting with friends.
"Toys are not really as innocent as they look. Toys and games are preludes to serious ideas.” - Charles Eames
Diversity of product use.
Lack of existing brand stigma.