CareerCup

i think they use the observer pattern.

In this each user is a subject and also behave as a observer to all of his frnds .

use hash table to find changes, use observer pattern to inform changes to all user who is publishing the article

when each such user "update" is generated its given some unique id. this unique id will act as a link to retrieve "update" specific data like a video, user comments.
Thus if a "update" unique id is duplicated by multiple users (friends) then we can see it as "User1" and "user2" posted this.

I would consider it more like a mail box scenario(message queue) where each user has a list of update from his friends sorted(and merged) based on time.
Whenever a user is making a "update", its like he is posting a message to his friends.

how about a priority queue or a splay tree?

It would definitely be done by Observable and Observer classes available in Java.
These objects should be kept in session scope.

are therr sampke questions for previsor online c++ tests