WhatsApp's appeal is simple: it does everything important (individual and group messaging, photo and video sharing and location sharing) really well, without ads, and for a very reasonable fee. For example you can currently get the app for free and you'll pay $0.99 a year thereafter.
If you're thinking "Facebook does all of that, and it doesn't cost anything" you're right but Facebook also does everything else.
It's bogged down in the aforementioned baby photos, people Like-ing things to try and win competitions, endless game invites, things you saw online last week, all kinds of irrelevant content and lots of ads. By comparison Snapchat and WhatsApp are much more focused and less ad-heavy: they do what you want and don't do what you don't.
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Doing it for the kids
Snapchat and WhatsApp aren't the only services with large teen appeal. Twitter usage is increasing among teen users, Instagram's doing well too, and negative newspaper coverage of Ask.fm doesn't appear to have done it any harm.
Teens are using Skype, Vine and Tumblr, Pinterest, Pheed and Instagram, with newer services such as diary.com hoping to capture some of this notoriously fickle demographic too.
The services that are doing well tend to fall into three categories: chat, creation and curation. Chat is self-explanatory; creation is posting stuff you've made yourself, whether that's a bathroom selfie or an Instagrammed burger; and curation is posting stuff that others have made, such as when you reblog a Tumblr image or retweet on Twitter.
Creation and curation are increasingly important. Pew reports that 54% of adult internet users are involved in online creation (up from 46% the previous year) and that 47% take part in curation, up from 41% the previous year.
Is Facebook fading?
It's important to keep all of this in perspective, though. Facebook has 1.19 billion active users and it's continuing to grow. However, its sheer scale means that even a tiny percentage of its user base represents an enormous number of people, and the more mature it becomes the more teenage users will want to use something else.
If you can tempt even a fraction of Facebook's teen users to spend time in your mobile app instead, you've got a nice little business.
The kind of business that can afford to turn down $3 billion dollars.