The WhatsApp instant messaging app has apologised to users after being unavailable for several hours on Saturday night.
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 Marwaha 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.