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."