Originally the #FollowFriday hashtag was used to recommend new tweeters to your followers. There would be one name and the reason you're recommending.
#FollowFriday @MarioDaCat because he always looks snazzy in his tuxedo.The reason is dat deyz a web page dat rates everybody, tells you who is your favorites, and even tells you whether a tweet counts toward ranking.
Tweets have to have #followfriday (#ff for short). Tweets divide up da point score between everybody in da tweet. More than ten #ff tweets gets ignored.
There are sites that show you who your best followers are so you can recommend them.