The California-based firm will have to pay WhatsApp USD 1 billion in cash and USD 1 billion in stock options in case it fails to close the deal in the stipulated time.
Facebook said the acquisition is subject to customary closing conditions, including regulatory approvals, and is expected to close in the second half of 2014.
"This date may be extended by us to August 19, 2015, if as of August 19, 2014 certain closing conditions applicable to Facebook (other than the receipt of certain regulatory approvals) have been satisfied," Facebook said in a US SEC filing last week.
The firm added that at present it expects that these conditions will be satisfied and that "we will extend the date to August 19, 2015."
"We have agreed to pay a termination fee to WhatsApp of USD 1 billion in cash and issue a number of shares of our Class A common stock equal to USD 1 billion, based on average closing price of the ten trading days preceding such termination, if the closing of this acquisition has not occurred by August 19, 2014," it said in the filing.
The filing further said: "... We currently expect these conditions will be satisfied and that we will extend the date to August 19, 2015."
The Mark Zuckerberg-led firm, which raised USD 16 billion in the richest tech sector public stock offering in 2012, said its biggest acquisition (of WhatsApp) so far, will not impact WhatsApp's brand.
The hefty transaction -- which includes USD 12 billion worth of Facebook shares, USD 4 billion in cash and USD 3 billion in restricted stock units - will boost the world's biggest social network presence by adding 450 million users of WhatsApp, including a significant number of youth.