"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.
Yesterday, some WhatsApp users found they were unable to connect to the app, while others complained their messages were not going through.
WhatsApp has not indicated how long the outage would last or when it began.
However, 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 after Facebook's announcement on Wednesday.
Facebook is betting huge on mobile with the eye-popping cash-and-stock deal for WhatsApp, which was only started five years ago but has quickly grown as a free alternative to text messages.
It is Facebook's biggest acquisition and comes less than two years after the California-based Internet star raised $16 billion in the richest tech sector public stock offering.
Neither WhatsApp nor Facebook were immediately available for comment.
However, 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!