Clearspring Technologies was founded in 2004 by Carnegie Mellon University graduate students Hooman Radfar and Austin Fath in Pittsburgh, Pennsylvania. The company began as an R&D and consulting concern focused on commercializing research concepts around the semantic web and social networks. Initial funding came primarily from grants and consulting projects, as well as from friends, family and a US$100,000 investment from Idea Foundry. Its first product was Semantic Start Page, a customizable, browser-based dashboard that displayed information and news. Users could add widgets from publishers across the web and share content across their social graph.

AddThis LLC was founded in 2006 by Dom Vonarburg. By 2007, AddThis had served more than 100 million widgets to websites, with website growth at 100 percent per month and some two million views a day.

In 2008, Clearspring acquired AddThis LLC, with the intent of creating a single content sharing platform for publishers under one brand: AddThis. The combined platform per Comscore was 254 million unique users.  Clearspring upgraded AddThis with widget-sharing capabilities from LaunchPad, then discontinued the LaunchPad offering, reaching 600 million unique users by the end of 2009.

In 2010, the company launched the Clearspring Audience Platform, a service for brand marketers to deliver interest-based display advertising across the web, which topped 1 billion unique users and was used by over 8 million unique domains.  Clearspring acquired data science company XGraph in 2011.  In September, the company hired a new CEO, Ramsey McGrory, formerly of RightMedia and Yahoo!, with co-founder Radfar becoming executive chairman.

On May 10, 2012, Clearspring changed its name to AddThis, its most widely used product. The company launched three sharing and analytics tools: Trending Content Box, Follow Tools, and Welcome Bar, as well as supporting content sharing for Pinterest and Web Intents.  In August, the company began offering social login.  In September, CFO Richard Harris took over as CEO.

In March 2014, the company was named number one on the Top 30 Syndicated Ad Focus Entities by comScore. 

On January 5, 2016, AddThis was purchased by Oracle Corporation for $175 million. Before being acquired, AddThis had raised roughly $73 million to date. 

The company was the subject of a lawsuit by Rembrandt Social Media, which also sued Facebook, for the use of patents that were first implemented in Surfbook and belonging to deceased Dutch programmer Joannes Jozef Everardus van Der Meer that involve the "Like" button.

• AddToAny
• List of social bookmarking websites
• Canvas fingerprinting
• ShareThis

• Official website

• Social bookmarking websites
• American companies established in 2004
• Internet properties established in 2004
• Companies based in Vienna, Virginia
• 2016 mergers and acquisitions
• Oracle acquisitions