The EU is to find out from service providers and technology firms if Facebook's $19 billion acquisition of WhatsApp would adversely affect the mobile messaging market and its customers.
Regulators have sent a long and detailed questionnaire to firms in a bid to find out if the deal would hold back innovation and lead to price increases.
In May, Facebook approached the EU to look at competition issues as WhatsApp's free service could rival existing European telecoms companies.
The deal would see the social network become one of the biggest players in in the mobile messaging market, and is seen as a test case in how EU competition law applies to social media sectors.
Depending on the feedback, the EU could launch a full competition review, demand concessions or wave the deal through.
Rivals were asked if the deal would impact advertising and whether mobile messaging users would be affected by the deal.
"If negative, please explain why and what kind of negative impact (for example price increase, deterrence of innovation, etc)," the document said.
The questionnaire also asked rival firms if they would face problems as a result of the deal.
"As a result of the Facebook/WhatsApp transaction, do you expect users of WhatsApp to face greater difficulties in switching to another consumer communications service/app?" the questionnaire asked.
If "As a result of the Facebook/WhatsApp transaction, do you expect that it will be more difficult for WhatsApp's competitors to expand their user base or to launch a new consumer communications service/app in the European Economic Area?"
The deal has already been approved by the Federal Trade Commission in the US, with the stipulation that WhatsApp adheres to current privacy practices.
