Dollar Tree said this week it is buying Family Dollar Stores for $8.5 billion or $74.50 a share. The combined company will have more than 13,000 stores and more than $18 billion in sales making it North America's biggest discount retailer. Meanwhile, real estate website operator Zillow is buying competitor Trulia in a $3.5 billion all-stock deal or about $70.53 a share. The transaction will make the combined entity the largest player in online real estate advertising. Finally, speaking of millions of users, Twitter said this week it had 271 million average monthly active users in its second quarter, a 24% increase from last year. Mobile users jumped 29% to 211 million.