Their chief rival is Realtor.com, owned by the National Association of Realtors, which claims to be "the number-one most visited homes-for-sale site" in the nation.
"This proposed merger doesn't change anything about the Realtor value proposition, and NAR remains committed to promoting and supporting the Realtor brand and Realtor value proposition to consumers," said the organization's spokesman, Adam DeSanctis. "In the Internet age, consumers are going to go online for real estate information, but they will continue to rely on local experts when they buy and sell property."
Of the $12 billion spent annually by real estate professionals on marketing, Zillow and Trulia, with headquarters in San Francisco, together receive about 4 percent of that total, they say.
To market listings on Realtor.com, agents must be Multiple Listing Service participants, and MLS must provide data and content to Realtor.com.
Agents and brokers pay a premium to the local MLS, and that allows the "flow" of for-sale listings to go on to Realtor.com.
Both Zillow and Trulia generate income by selling advertising.
John Duffy, president of Duffy Real Estate in Narberth and St. Davids, said he doubted the transaction "will have any effect, as it is just one information company buying another."
"Buying up the competition sounds very familiar in our industry," he said.
Martin Millner, of Coldwell Banker Hearthside in Yardley, said there was "concern within the real estate community [about the] effect it would have if Zillow actually got into the brokerage business."
"For now, they have given no indication that they are considering that, but who knows," Millner said.
"Large mergers like this do cause some things to change," said Weichert Realtors' Noelle M. Barbone in Media.
"However, knowing what their motives are behind it - other than profits - only time will tell," Barbone said.
Center City Realtor Mark Wade, of BHHS Fox & Roach Realtors, said he could not imagine that the merger would have any effect, "since they will still operate as two different companies."
Wade said the real question is, "What's in a name?"
He quickly answered: "I would guess not much."
"The move in the online real estate industry is toward mobile," Wade said, "and if they can dominate that segment, I think they will do well."
At a Glance
Chief executive officer
Seattle San Francisco
Unique users in June
83 million 54 million