CHICAGO (Reuters) - Users of WhatsApp reported on Saturday that the rapidly expanding mobile messaging app was down, just days after its acquisition by Facebook for $19 billion.

"Sorry we currently experiencing server issues. We hope to be back up and recovered shortly," WhatsApp said in a tweet to its more than 1 million Twitter followers on Saturday around 4p.m. EST (2100 GMT).

Five-year-old WhatsApp currently has about 450 million users globally and has been adding a million daily.

Some of those users took to other forms of social media, including blogs, on Saturday to report the outage and vent their frustration.

Facebook did not immediately respond to requests for comment.

(Reporting By Dave Gregorio)