From Google's Sydney offices. Image source: Google.

When billionaires sell shares, they tend to make waves. The Motley Fool would never advise you to follow those market-movers' every footstep, but it certainly can't hurt to watch where the big money is flowing.

Last quarter, 18 billionaire traders added a total of $2.6 billion to their collective holdings in Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL). Sure, that's only 0.5% of the Google parent's massive market cap, but some of these fresh bets by proven master investors are very significant in context of each buyer's current portfolio.

In particular, the four biggest Alphabet buys made me wonder whether I should be adding to my own position sometime soon. Here's what I found.

These billionaires are betting big on Alphabet right now

In a review of recent Form 13F filings, where hedge fund owners and other billionaires report their trades on a quarterly basis, several well-known hedge funds were seen adding to their Alphabet holdings -- or even kick-starting brand new positions from scratch.

Risk management expert Bruce Kovner's Caxton Associates pumped another $273 million into its Alphabet holdings during the quarter. That's an 89% increase from the last report, three months earlier. Big Caxton buys can be seen as an endorsement of the stock's low risk profile. Some investors would not apply that "low risk" label to Alphabet, and that's fine. In my view, it's exactly the kind of stock you can buy now and keep for twenty years or more, never losing any sleep over Google and its satellite businesses.

Then we have Glenview Capital Management, under the wing of billionaire Larry Robbins, increasing its Alphabet stake by a modest 10.6%. But Glenview already has a huge holding of this stock, so it's still an impressive $150 million investment that brings its total stock ownership to $1.4 billion. Robbins is an unusual hedge fund manager because he likes to buy and hold for the long term, looking for solid growth stories at reasonable prices.

For example, Glenview bought a lot of hospital stocks as Obamacare inched toward final approval, and made a killing on that correct investment thesis. Many funds would have sold off those stakes by now, but hospital chains and health insurance companies still make up the largest holdings in Glenview's portfolio.

And don't forget about Leon Cooperman, whose Omega Advisors fund scooped up $184 million worth of Alphabet shares this quarter. The stock used to account for 1.3% of the fund's total holdings, but this move lifted Alphabet to a 2.5% share of invested assets. It's now the single largest holding in Omega's portfolio. Numero uno, folks.

Cooperman prefers hand-picking growth stocks, despite the risks involved in choosing growth stocks in an eternal minefield of potential duds. Cooperman's motto is "any stock or bond at the right price," and his investing style is often compared to Warren Buffett's but without Buffett's single-minded pursuit of industries he knows inside and out.

Omega's investment balances out Glenview's low-risk badge with an endorsement from a fund with a much higher risk tolerance and larger appetite for growth potential. When both of these funds are chasing the same stock, it points to a tantalizing blend of stability and growth potential. Like I said, that's exactly what I see happening in Alphabet.

This particular investment feels a little bit tainted due to Omega's repeated involvement in SEC investigations, including a $500,000 fine for alleged bribery in 1998. You could write Cooperman's opinion off like spoiled meat, or you could forgive, forget, and analyze his moves anyway. Your choice -- just making sure you're not ignoring this elephant in the room.

Let me also point out that Pennant Capital Management built an Alphabet investment from zero to $534 million during this quarter. It's the largest Alphabet increase among the hedge fund moves in our study, and the stock shot from nowhere to the fifth largest holding in Pennant's portfolios.

Unfortunately, little is known about Pennant's investment style, since the service is closed to new investors and management is keeping details close to the vest. We do know that fund leader Alan Fournier cut his investing teeth under David Tepper at Appaloosa Management, which specializes in turnaround stories and broken stocks in need of a fix. Neither thesis seems to apply to Alphabet, so the Tepper connection is probably just a red herring.

Who else is buying Alphabet shares?

Alphabet had a great year in 2015, with share prices rising more than 42%. So far, 2016 hasn't worked out quite that well, and the stock is underperforming the broader market. These billionaires are not simply going with the flow, but placing big bets while other investors are showing less conviction.

Digging into the business, Alphabet is riding the twin trends of mobile search and online video viewing to strong revenue and earnings results. The company doesn't always beat Wall Street's targets, but Alphabet doesn't publish forward guidance around which analysts could build their financial models.

Meanwhile, Alphabet shares have backed down to P/E and cash flow valuation ratios that would feel familiar to a time-traveling investor dropping in from 2013. And the Alphabet of 2026 will look very different from the one we see today, as it's evolving into a conglomerate with lots of operations not based in cyberspace.

For all these reasons, I see why both risk-hater Larry Robbins and growth-lover Leon Cooperman are diving deeper into Alphabet right now. Between them, they're giving it a ringing endorsement.