On a - I have no idea.
On b-
Not sure if it's worth $25 billion, but I have noticed that it is the "Facebook" for younger generations, and that when they use it, they use it all the time, not just a few times a day like other social media. It's a way to share their lives - in real time - with their friends and followers, and have their friends do the same.
(Look at me sounding like an old man!)