WhatsApp, which has more than 450 million monthly users, said it was having “server issues”.
The company suffered the outage just days after it was snapped up by Facebook for $19bn (£11.4bn).
The messaging service tweeted at 8.16pm UK time on Saturday: “We hope to be back up and recovered shortly.”
Users reacted angrily, saying their chat conversations were only showing a loading asterisk and the alert “Connecting…”
Taylor ITWT16 said on Twitter: “As Soon As Facebook Buys Whatsapp, Whatsapp Starts Malfunctioning.”
Jazzy tweeted: “Its been like 4 hours, you would have thought they could sort whatsapp out by now.”
By 10.30pm the service appeared to be working again.
The firm tweeted: “WhatsApp service has been restored. We are so sorry for the downtime.”
Facebook announced last Thursday that it would pay $4bn (£2.4bn) in cash and $15bn (£9bn) in Facebook shares as part of the deal to buy the real-time messaging service.
The app’s founders and employees will get $3bn (£1.8bn) of the shares as restricted stock that will vest over four years after the deal closes.
The purchase marks the largest single acquisition in Facebook’s 10-year history.
WhatsApp will “continue to operate independently and retain its brand” despite the acquisition, Facebook said.
Founded by a Ukrainian immigrant who dropped out of college, Jan Koum, and a Stanford alumnus, Brian Acton, WhatsApp is a Silicon Valley startup fairytale.
The acquisition will also see Mr Koum – a former Yahoo! engineer – join Facebook’s board of directors.
Facebook is known to make bold moves to thwart competitors – it famously bought Instagram for $1bn after a weekend of negotiations.
