One of the main reasons Facebook 'won' was because it managed to beat MSN.
MySpace had a chat feature. Bebo had a chat feature. But MSN still reigned supreme as the way to talk to people online. MySpace and Bebo had bulky inboxes like an email client and MSN was quick and fun. MSN is still slightly better for the user than Facebook ever was; it did webcam chat without issue much earlier, you could appear offline to 'friends', you could add people you barely knew and satisfactorily block them forever.
So then Facebook comes onto the scene and has integrated chat. You've added all your friends, like on Myspace and Bebo, and suddenly you don't _need_ MSN up to talk to them. You still do for the few friends you have that don't have Facebook, but slowly it becomes easier to just open Facebook and open MSN when you want to talk to those friends. Eventually MSN just remains closed. And as it was suffering from the iTunes style of updating anyway, that wasn't the worst of things. Even if it was slightly better, the extra effort isn't worth it.
Facebook damn well knows this. They've got you talking on their website, now they've got you on their apps. They are an important part of life because they've also beaten texting in how we communicate.
So Snapchat comes along, and Facebook don't like it. There's another way to communicate, and this one is even more low effort than before. It's seamless, it's easy to do and to understand. They know that one of the reasons they're so dominant is that the price to communicate is being on Facebook.
So why is Snapchat so dominant, aside from ease of use and tumblr-esque simplicity in communication? You're right that it captures attention and demands you look at every pixel for those 3-10 seconds. You've probably also heard that it's used for porn, but it's a method of communication so that's a given.
Taking a DP on Facebook, for me, may take up to 2 hours. I'm not actually kidding. I'm not an expert photographer, and I'm damn well not that ugly or attractive to merit it, but I like taking pictures of myself for Facebook that make me look okay. I feel compared to everyone else I know and people I don't know.
Snapchat, if you trust it, takes your photo and then removes it forever. Okay fine it's not foolproof, but the idea is that you send it to your friends and assume they probably aren't horrible people. People share things with someone because they can, and in this system they can't. You get a sense of impermanence that means you don't worry as much.
If I upload a random photo of my day on Facebook, maybe 1-3 of my friends will give a shit, so I wouldn't put it on FB, but I also can't be bothered uploading it to send to them in a message. So Facebook photos are now where you put the highlights of yourself to show off. Snapchat is where you can fuck around with friends.