This is an example implementation of a really simple algorithm in Ruby to identify trending words and phrases in a collection of posts. This was written in a single sitting, late at night, without research, so it's definitely not the most efficient way to tackle this problem. I haven't even tested it with real data to make sure that it works as …

simple_trending_example.rb

# constant array with the 100 most common words in English
COMMON_WORDS = ["the","be","to","of","and","a","in","that","have","i","it","for","not","on","with","he","as","you","do","at","this","but","his","by","from","they","we","say","hew","she","or","an","will","my","one","all","would","there","their","what","so","up","out","if","about","who","get","which","go","me","when","make","can","like","time","no","just","him","know","take","people","into","year","your","good","some","could","them","see","other","than","then","now","look","only","come","its","over","think","also","back","after","use","two","how","our","work","first","well","way","even","new","want","because","any","these","give","day","most","us"]

# these numbers are totally made up, so you'll probably want to tweak them
MINIMUM_FREQUENCY_FOR_THREE_WORD_PHRASES = 10
MINIMUM_FREQUENCY_FOR_TWO_WORD_PHRASES = 20
MINIMUM_FREQUENCY_FOR_SINGLE_WORDS = 30

three_word_phrases = Array.new
two_word_phrases = Array.new
single_words = Array.new

# gets the 100 most popular post using a popular scope, presumably sorting by views/reposts/etc in the last 24 hours or something
most_popular_posts = Post.popular(100)

# for each of the 100 most popular posts...
most_popular_posts.each do |post|
  # for all of the three word phrases in each post...
  post.scan(/\b\w+\s\w+\s\w+\b/).each do |phrase|
    # normalize the phrase into lower case
    phrase = phrase.downcase
    # put each word of the phrase into an array
    words_in_phrase = phrase.scan(/\b\w+\b/)
    # unless at least two of the words in the phrase are common...
    # this allows phrases like "the big apple" but not "in the big"
    first_word_is_common = COMMON_WORDS.include? words_in_phrase[0]
    second_word_is_common = COMMON_WORDS.include? words_in_phrase[1]
    third_word_is_common = COMMON_WORDS.include? words_in_phrase[2]
    unless first_word_is_common and second_word_is_common or \
           first_word_is_common and third_word_is_common or \
           second_word_is_common and third_word_is_common then
      # if the phrase already exists in the list...
      index_of_phrase_if_found = three_word_phrases.index {|hash| hash[:string] == phrase}
      if index_of_phrase_if_found then
        # increment the frequency of the phrase by 1
        three_word_phrases[index_of_phrase_if_found][:frequency] += 1
      # otherwise the phrase isn't already in the list...
      else
        # add the phrase to the list with a frequency of 1
        three_word_phrases.push({ string: phrase, frequency: 1 })
      end
    end
  end
end

# create a new array excluding the three word phrases that aren't frequent enough to be trending
trending_three_word_phrases = three_word_phrases.select { |hash| hash[:frequency] >= MINIMUM_FREQUENCY_FOR_THREE_WORD_PHRASES }

# then the same thing for two word phrases, but with an expensive twist
most_popular_posts.each do |post|
  post.scan(/\b\w+\s\w+\b/).each do |phrase|
    # normalize the phrase into lower case
    phrase = phrase.downcase
    # put each word of the phrase into an array
    words_in_phrase = phrase.scan(/\b\w+\b/)
    # unless at least one of the words is common
    unless COMMON_WORDS.include? words_in_phrase[0] or COMMON_WORDS.include? words_in_phrase[1] then
      # for each of the three word phrases...
      trending_three_word_phrases.each do |larger_phrase|
        # if the two word phrase is part of an existing three word phrase, skip to the next phrase
        next if larger_phrase[:string].include? phrase
      end
      # if we've made it this far, the phrase has met our criteria, so let's count it
      index_of_phrase_if_found = two_word_phrases.index {|hash| hash[:string] == phrase}
      if index_of_phrase_if_found then
        # increment the frequency of the phrase by 1
        two_word_phrases[index_of_phrase_if_found][:frequency] += 1
      # otherwise the phrase isn't already in the list...
      else
        # add the phrase to the list with a frequency of 1
        two_word_phrases.push({ string: phrase, frequency: 1 })
      end
    end
  end
end

# create a new array excluding the two word phrases that aren't frequent enough to be trending
trending_two_word_phrases = two_word_phrases.select { |hash| hash[:frequency] >= MINIMUM_FREQUENCY_FOR_TWO_WORD_PHRASES }

# and again for single words
most_popular_posts.each do |post|
  post.scan(/\b\w+\b/).each do |word|
    # normalize the word into lower case
    word = word.downcase
    # unless the word is common
    unless COMMON_WORDS.include? word then
      # this part is tricky. i *think* it's a good idea, but i'm not sure.
      # the idea is that if "michael jackson" is trending, "michael" shouldn't
      # also show as trending, as it's out of its meaningful context
      trending_three_word_phrases.each do |larger_phrase|
        # if the word is part of an existing three word phrase, skip to the next word
        next if larger_phrase[:string].include? word
      end
      trending_two_word_phrases.each do |larger_phrase|
        # if the word is part of an existing two word phrase, skip to the next word
        next if larger_phrase[:string].include? word
      end
      # if we've made it this far, the word has met our criteria, so let's count it
      index_of_word_if_found = single_words.index {|hash| hash[:string] == word}
      if index_of_word_if_found then
        # increment the frequency of the word by 1
        single_words[index_of_word_if_found][:frequency] += 1
      # otherwise the word isn't already in the list...
      else
        # add the word to the list with a frequency of 1
        single_words.push({ string: word, frequency: 1 })
      end
    end
  end
end

# create a new array excluding the words that aren't frequent enough to be trending
trending_single_words = single_words.select { |hash| hash[:frequency] >= MINIMUM_FREQUENCY_FOR_SINGLE_WORDS }

# now you have three collections of trending words and phrases. from here you
# can play with them however you like - keeping them separate or combining
# them. you can choose to sort alphabetically for a word cloud or by frequency
# for other visualizations. have fun. :-)