By virtually every measure, both Facebook (META -1.08%) and its professional networking counterpart LinkedIn (LNKD.DL) have unlimited upside as the world transitions to digital. In the U.S. -- the world's largest advertising market -- digital spots are expected to surpass longtime mainstay TV this year with an estimated $72 billion in sales.

In LinkedIn's case, advertising revenue is a distant second to its Talent Solutions sales, but that equates to a significant opportunity in the years ahead. As dominant as Facebook already is, it still manages to report stellar growth quarter-in and quarter-out as its share of social media ad spend swells. So, which is the better buy, Facebook or LinkedIn?

Image source: LinkedIn.

The case for LinkedIn

As the professional networking king demonstrated in last quarter's earnings release, it's growing across all of its key units. Led by a 24% jump in talent division revenue to $623 million, LinkedIn's total sales climbed 23% year over year to $960 million.

LinkedIn's job-related revenue led the way, but its online learning push is showing signs of life, climbing to $67 million last quarter. Along with its 26% jump in Marketing Solutions revenue to $175 million and a 17% year-over-year gain in Premium Subscription sales to $162 million, those are baby steps in the right direction in terms of diversifying LinkedIn's reliance on job hunters.

It wasn't long ago -- in February of this year -- that LinkedIn stock nosedived 30% overnight on "poor" guidance. It's safe to say those days are long gone. Certainly news of Microsoft's (MSFT 0.25%) $26.2 billion deal for LinkedIn drove its share price up near the $196 acquisition price, but CEO Jeff Weiner's focus on utilizing artificial intelligence (AI) to boost both ad revenue and online learning sales was already paying dividends.

That said, joining forces with Microsoft should not only energize sales of its Skype, Outlook 365, and other offerings, its advanced AI and data analytics capabilities could prove to be a gold mine for LinkedIn. Ad sales are a huge opportunity LinkedIn is just beginning to target, and tossing in Microsoft's strengths and marketing could prove to be even more of a long-term growth driver than job seekers.

Image source: Facebook.

The case for Facebook

Pundits and investors expecting what would seem to be the inevitable slowdown of Facebook's growth were sadly disappointed again last quarter. Due in part to "making progress putting video first," Facebook reported a mind-boggling 59% improvement in ad sales in its third quarter, accounting for $6.8 billion of its $7 billion in total revenue.

As impressive as Facebook's revenue gains are, even those take a back seat to its stellar user growth. As far back as 2013, "concerns" were running rampant about "Facebook fatigue" among its 1.11 billion monthly average users (MAUs). Fast-forward to today, and Facebook boasts more daily users -- 1.18 billion of its 1.79 billion MAUs -- than monthly users back when saturation concerns were running high.

Mobile was also cited as a Facebook shortcoming over the past couple of years. Today, mobile spots equated to 84% of Facebook's ad revenue last quarter, up from "just" 78% a year ago. As for spending, Facebook CFO David Wehner had forecast an increase of over 50%-plus this year, but thanks to overhead climbing just 28% in Facebook's third quarter, net income climbed 166% to $2.38 billion.

While Facebook's stellar earnings and user growth warrant optimism, better still are the multiple untapped revenue opportunities waiting in the wings. WhatsApp is in the early stages of becoming monetized, Messenger's more than one billion MAUs remain on the back burner, and Workplace along with Facebook's Rift virtual reality (VR) headset are still in the early stages of growth.

So, which is the better buy, LinkedIn or Facebook? Thanks to its fairly clear roadmap for growth, particularly compared to LinkedIn's relatively uncertain, though promising, future with Microsoft, Facebook earns the nod.