Zillow Inc. agreed to purchase Trulia Inc. for $3.5 billion in an all-stock deal that is designed to make the combined entity the largest in online real estate advertising.

Trulia shareholders will receive 0.444 shares of Zillow for each share of Trulia, the companies said Monday in a statement. That’s equivalent to Zillow offering $70.53 a share for Trulia, or 25 percent above Trulia’s Friday closing price of $56.35. Zillow shareholders will own two-thirds of the combined company, while Trulia stockholders will own a third.