New York--Just weeks after announcing its plans to purchase Abacus Direct Corp., online advertising network DoubleClick announced Tuesday that it has agreed to acquire rival NetGravity in a stock transaction valued at $530 million. DoubleClick reported that its second-quarter revenue reached $44 million, up 154% from a year earlier. Net profit figures were not yet available. DoubleClick lost $18.1 million in 1998 on revenue of $80 million. ``Our goal has been to build the broadest possible infrastructure for Internet advertising,'' Kevin O'Connor, DoubleClick's CEO, said in a statement.