EU regulators are attempting to identify possible problems of the deal, including price hikes and effects to innovation.
Regulators will speak to Facebook rivals and telecoms operators in the region, after Facebook sought approval from the EU. By acquiring WhatsApp, Facebook will now directly compete with telecoms operators in the mobile messaging market.
The European Commission is due to provide its decision by October 3, and will clear the deal unconditionally, demand concessions or start proceedings into a wider probe.
EU regulators have reportedly sent a questionnaire to relevant companies, and asked them to provide opinions on whether mobile messaging could potentially substitute traditional voice calls, text messages and emails.
Facebook is due to close the deal this year, after the US Federal Trade Commission cleared the acquisition in April.
