ProgrammableWeb has been tracking Twitter mashups since the first one was added in December, 2006. Looking at the number developers have added to the database, one thing is clear: 2009 was a huge year for Twitter and apps built upon it.
The above graph shows the number of mashups added by month, from the beginning of 2007 through last month. You can see a marked increase in 2008, followed by tremendous growth in 2009. More Twitter mashups were added in April 2009, than all of 2007.
Many of the mashups have been "filling holes," as Twitter investor Fred Wilson described. Many developers reacted negatively to the phrase, but it's one of the better stories of a development platform. For example, Nearby Tweets (original screenshot from our Nearby Tweets mashup profile pictured below) launched a year ago, several months before Twitter announced geolocation.
Though three months of data is not quite enough to establish a pattern for 2010, there have certainly been fewer mashups added per month, perhaps because so many ideas have already been executed several times over. As Twitter gears up for Chirp, its first developer conference, there have to be questions about where to go from here. Wilson suggested several places ripe for "killer apps," including social gaming, enterprise and analytics.