What: Shares of social networker Facebook (NASDAQ:FB) have soared today, up by 15% as of 12:20 p.m. ET, after the company reported strong fourth-quarter earnings.

So what: Revenue in the fourth quarter jumped 57% to $5.8 billion, driven by continued growth in advertising sales. Adjusted net income came in at $2.3 billion, or $0.79 per share. Both top- and bottom-line results utterly crushed the consensus estimates of $5.4 billion in sales and $0.68 per share in adjusted earnings.

Now what: Facebook continues to grow its user base, which now stands at 1.59 billion monthly active users. Of these users, 1.44 billion are mobile monthly active users. The company generated nearly $2.1 billion in free cash flow during the quarter, and $6.1 billion in free cash flow throughout all of 2015. Mobile ad revenue comprised 80% of all advertising revenue in the fourth quarter, as Facebook continues to execute impeccably on the mobile front.