I am sure you know about Facebook Timeline and the idea behind it: the history of your entire life in Facebook will be exposed. A great way to see what you did last year, a very risky proposition for people that change girlfriend or boyfriend: your new girlfriend will be able to go back in time and see what you wrote about your old girlfriend. It should be a lot of fun...
The change seems small, since it is just your profile who got larger (and you probably could get that information anyway, clicking on Older Posts). However, I believe the change in paradigm is massive.
Twitter has always been about NOW. Instant flow of messages. Anything older than a few seconds is history.
Despite actually having a memory (I am sure Twitter has every tweet you ever did), the way it is built and presented makes history irrelevant for a user (not for Twitter, that is gold). You can write pretty much what you want, and you know it will soon be forgotten.
Knowing a site has no memory - or shows no memory - makes the interaction completely different. More casual. You write things right in the moment. You do not think what you are doing is written in stone and will be there forever to haunt you one day.
Facebook had nothing like Twitter at the beginning. When Twitter became hot, they added status updates. It became like Twitter, but just for "friends". Then they added a public option (not the Obamacare one), mostly to follow Google+ (yep, they are smart, copying smart ideas is being smart) so it matched Twitter perfectly.
Then they added Timeline.
No actual changes anywhere in the site, just in your profile. You can post a public story, and it is like Twitter.
However, it is Twitter with a memory. A visible one. A searchable one. Not just by you, by anyone who knows you. Or will eventually know you in the future.
The change is dramatic. When you post in Facebook, you are now conscious of it. I am not talking about you geeks, I know you knew it. I know you were aware anything you did was permanent. You know what a database is and how to retrieve anything with a SQL statement. I am talking about the rest of us, the other 99.9999%, the real people.
They thought Facebook was like Twitter. They thought Facebook had no memory. They now have a visible confirmation it is not like that. Facebook has memory. You should not post anything assuming it will disappear with time. It is so clear, so obvious. Every single Facebook user will know it. And they will think one or two seconds more about what they are about to post.
Facebook and Twitter are now miles apart.
I am not sure this is going to be that good for Facebook. Having users worrying about posting might not be such a great idea. We'll see. I sure am glad that I always treated Facebook exactly like Twitter ;-)