According to an algorithm, the 4 ingredients required are congruency, emotive strength, network involvement, and something called "paired meme synergy"
Interesting! Think this algorithm may be highly applicable to creating enticing and engaging e-learning apps (especially those that have a social sharing element)...
Story Bumping: Stories you haven’t seen yet because they were “below the fold,” on News Feed are eligible to be bumped up further in News Feed the next time you check Facebook.
Last Actor: Facebook will take into account the last 50 engagements of a user, giving more weight to people and pages the user has recently interacted with.