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. :: Watch Sky News live on television, on Sky channel 501, Virgin Media channel 602, Freeview channel 82 and Freesat channel 202.
Baada
ya mtandao wa mawasiliano ya ujumbe mfupi wa simu WhatsApp kupata
tatizo kwa masaa machache watumiaji wa mtandao huo wameingia kwenye
mitandao tofauti na kuanza kulalamika kuhusu facebook kununua mtandao
huo na kuamua kufunga huduma yake ili facebook itumike zaidi. Fahamu
mmiliki wa facebook amelipia dola bilioni 19 kununua mtandao huu.
Kupitia twitter account ya whatsup walitoa taarifa kuwa walipata
tatizo kubwa na wanafanya marekebisha na kuwa mtandao huo utarudi hewani
baada ya muda mfupi.
Hizi ndio twit zao
Mambo haya yalianza kusamba kwenye mitandao kuhusu whatsup kuzimwa.