See also "A Brief History of NetGravity":
In October 1999, DoubleClick purchased NetGravity in a stock deal valued at $530m. The NetGravity product line is now completely incorporated into the DoubleClick portfolio of advertising management products. The original founders and management team have all left to pursue new opportunities. NetGravity, having helped to create the multi-billion dollar internet advertising market, has essentially ceased to exist.
-- https://news.ycombinator.com/item?id=41206733
DoubleClick rebranded NetGravity AdServer as DART Enterprise... On March 11, 2008, Google acquired DoubleClick for $3.1 billion. In June 2018, Google announced plans to rebrand its ads platforms, and DoubleClick was merged into the new Google Marketing Platform brand.