Since being spun out from eBay in 2015, PayPal Holdings, Inc. (NASDAQ:PYPL) has been rapidly growing its business. The company's sales are skyrocketing, transaction volume on the company's online and mobile payment platforms has spiked, and the company's share price has jumped about 170% over the past three years.

All of that share price growth may have investors wondering if PayPal's run is finishing up or just getting started. If we take a closer look at the current growth trends for the company, it's clear that PayPal is building its business into a payment powerhouse that could be a great long-term investment for many more years to come. Here's why.

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Strong customer growth and increasing engagement

One of PayPal's bright spots is its ability to grow its customer base, and how it's building new features to keep them engaged.

PayPal grew its active customers by 7.7 million in the second quarter of this year, an increase of 18% from the year-ago quarter. This brings the company's total active users to 244 million, and those users are tapping into PayPal's platform more than ever before. Payment transactions per active account reached 35.7 in the second quarter, which was up 9% year over year.

PayPal's management said on the earnings call that they're expecting more customer growth over the next two quarters and expect to add more than 30 million net new active customer accounts for the full year.

CEO Daniel Schulman said on the call that PayPal's growth and engagement comes from the company's customer choice initiatives, its growing partnerships with merchants, and its commitment to its customers. Schulman's mention of customer choice refers to a strategy the company implemented earlier this year that allows PayPal users to easily choose what credit or debit card they want to use for a specific transaction. He mentioned that 45 million customers have already adopted the feature and that it's led to "calls per transaction into our contact centers are now at their lowest level since we began tracking this metric more than seven years ago."

Schulman also noted that PayPal's expanded partnerships, including a new partnership with Alphabet's Google, has helped drive customer growth. For example, PayPal users who use the payment service for one of Google's services, like Google Play, will soon be able to pay "across the Google ecosystem," including the Google Store and YouTube.

Finally, PayPal's management believes the company's strategy of being a "customer champion" will help to continue growing the company's users. "Our focus on putting customers first and delivering exceptional experiences has created meaningful customer trust and deep loyalty to the PayPal brand," Shulman said.

Payment dominance and a massive addressable market

According to a recent report from Third Point analyst Daniel Loeb, PayPal process between 20% to 30% of all e-commerce transactions, excluding transactions in China. Not only is that a heck of a lot of transactions that PayPal's processing, but the company's checkout conversion rate of 89% is nearly two times higher than that of credit and debit cards as well, according to Loeb.

Analyst figures aside, PayPal has proved with its own numbers that its users are using the platform more and more. Payment transactions jumped by 28% to 2.3 billion in the second quarter, and the company's total payment volume (TPV) was up 29% year over year to $139 billion.

The current TPV is nothing to sneeze at, but PayPal's management is confident that the company has a massive $100 trillion total addressable market (TAM) in the payment space that it's only begun tapping into.

"And we have a very small share of that total TAM, so it's very early innings. And that's why we said at our Investor Day that we think our greatest potential lies ahead," Schulman said.

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Mobile and peer-to-peer payments

PayPal's growth expands far beyond the company's traditional online transactions. The company's peer-to-peer payment (P2P) mobile app, Venmo, grew its transaction volume by 50% in the second quarter, to $33 billion, and accounted for nearly a quarter of the company's total payment volume.

The growth from Venmo is important because the mobile P2P market is expected to grow from $157 billion in transaction volume this year to $244 billion by 2021.

But P2P transactions are also included in the broader mobile payment market, and PayPal is benefiting there as well. The company processed about $54 billion in mobile payment volume in the quarter, up about 49% from the year-ago quarter. That means about 39% of the company's overall TPV comes from mobile payments. That's good news as merchants are increasingly looking to serve customers with ways to pay for goods using their smartphones, and even better because the mobile payment market is expected to grow from $601 billion in 2016 to $4.5 trillion by 2023.

The verdict? PayPal is a buy

When you add up the company's user growth, the fact that its transaction volume is skyrocketing, and its ability to grow its traditional platform and its mobile payment platform, PayPal looks like a compelling buy. The company has a lead in the e-commerce transaction market, a massive pile of customers who are engaged with the platform, and it's building out its future with P2P and mobile transactions.

PayPal's stock trades at about 32 times the company's forward earnings right now, which is just slightly less than the tech industry's average price-to-earnings ratio of about 33. That means the company's stock isn't all that expensive, relative to its peers, even after its massive share price gains over the past few years.

Of course, PayPal faces competition just like any other company, and investors should keep a close eye on Square to ensure that PayPal continues to outpace this rival in the mobile payment space. Still, I think PayPal's early leading in payment processing, its ability to build strong merchant relationships, and its growing opportunity in mobile make the company a solid long-term payment play.