Three days after being purchased by Facebook, WhatsApp's 450 million worldwide users were unable to access the smartphone free-messaging service for several hours on Saturday, AFP reports.
"Sorry we currently experiencing server issues. We hope to be back up and recovered shortly," WhatsApp said in a message on Twitter that was retweeted more than 25,000 times in just a few hours and provoked ridicule because it comes so soon after Facebook's hefty acquisition.
Some WhatsApp users found they were unable to connect to the app, while others complained their messages were not going through, according to AFP.
The specialist website techcrunch.com suggested the problem might be down to "a surge of signups and usage that has overloaded its servers" after the publicity the app garnered following Facebook's announcement on Wednesday.
Neither WhatsApp nor Facebook were immediately available for comment, but the outage was the source of much amusement - as well as anger - on Twitter.
"I expect you are all away from your desks on the ales drinking some of that Facebook cash!" one user, "leonclarance," replied to WhatsApp's tweet about the blackout.
Another simply tweeted, "Turn down Facebook's offer guys!"
On Wednesday, Facebook said it was buying WhatsApp for more than $16 billion in cash and stock.
The purchase includes $12 billion in Facebook shares and $4 billion cash. It calls for an additional $3 billion in restricted stock units to be granted to WhatsApp founders and employees that will vest over four years.
WhatsApp is a cross-platform mobile app which allows users to exchange messages without having to pay telecom charges.
Several months ago, Facebook acquired Israeli startup Onavo, which developed an application to help users save money on internet data connection charges.