Facebook closed its acquisition of mobile messaging service WhatsApp on Monday, with the final price tag reaching the $22 billion because of the increased value of Facebook's stock in recent months. WhatsApp founder Jan Koum will receive nearly $2 billion in stock, vesting over a four-year period, as an inducement for him to stay with the company, according to a regulatory filing on Monday.

The acquisition recently received regulatory approval in Europe.

WhatsApp has more than 600 million monthly users.