What does #followfriday mean in Twitter?
Perhaps, every Friday you see tweets including #followfriday (or shortened #ff) and usernames preceded by the @ symbol. For example:

#followfriday for the e24apps.com founders @aartiles24 @javierburon @luispablopardo
You may wonder what those #followfriday on Twitter are. This tradition was born spontaneously within the Twitter community to recommend your favorite tweeps to your followers. This portal counts all of these recommendations and computes a ranking of the most recommended Twitter users in the world.
Let's look at a bit of the history. Everything began on the 16th of January, 2009 with a tweet from Micah Baldwin (@micah):

I am starting Follow Fridays. Every Friday, suggest a person to follow, and everyone follow him/her. Today its @fancyjeffrey & @w1redone.
And a couple of minutes later Mykl Roventine (@myklroventine) proposed the creation of the #followfriday hashtag:

@micah Great idea! You need a hashtag for that - #followfridays
From the very beginning the #followfriday movement became a phenomenon comparable to Twitter itself. We could say that #followfriday is one of the most popular hashtags on Twitter.
A hashtag is a keyword preceded by the # symbol
When we write a hashtag preceded by the # symbol we create a hashtag. Hashtags are very useful to categorize tweets of the same topic making this topic easy to follow as a channel of tweets. In this way we can follow in real time all the tweets that include the selected hashtag using Twitter's search feature. For example, we can see in real time all the #followfriday recommendations.
Over time #followfriday recommendations have been overused and lost their value. Every Friday we receive hundreds of recommendations and all of these are impossible to check out and follow. That's why we have created www.followfriday.com, to generate a ranking based on these recommendations and make it easier to follow the most recommended tweeps with only one click without the need to visit profile by profile.
Global Rankings
FFValidator
Want to check if a tweet was valid for the FollowFriday Ranking and how much it worth for each recommended user? Drag "FFValidator Bookmarklet" to your browser's bookmark toolbar or add the link to your bookmarks.

