Mitchell Trubisky, the starting quarterback for the Chicago Bears, had a solid season, leading his team to a 12-4 record and its first division title since 2010. To show his appreciation, Federico Prian, a 24-year-old Chicago native, started sending Trubisky $8 — roughly the cost of a beer — on Venmo for each win.

"I'll buy my friends a beer to be nice, so I'll send Mitch a beer any day," Prian told CNBC in December, prior to the Bears' playoff loss last week to the Philadelphia Eagles. "My dream would be for him to thank me and say I kept him going."

Venmo gained popularity as a peer-to-peer payments app conveniently used for sending rent to a roommate or splitting the check at dinner. But Prian is among a subset of Venmo fanatics using the PayPal-owned service as a unique link to their favorite celebrities and a way to — quite literally — pay their respects. Venmo users have transferred tips to "Saturday Night Live" cast member Michael Che as well as John Graham, a former contestant on ABC's "The Bachelorette."

Graham, one of Venmo's earliest engineers, was flooded with fan payments after he was sent home from the reality show last year. Women across the country sent Graham, who earned the nickname "Venmo John" after he appeared on the show, two cents — a play on the phrase.

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It's a kitschy use case, to be sure, but the love that youngsters are showing Venmo is a big boon for PayPal. The company's sales are rising in the double digits, the stock has climbed 13 percent in the past year and CEO Dan Schulman said at an investor event in December that Venmo is "on fire," with 78 percent year-over-year growth in terms of volume.

Venmo was founded in 2009 and acquired by Braintree in 2011. PayPal then bought Braintree two years later, snapping up Venmo in the process. Unlike PayPal, which also offers an easy way to send money to friends, Venmo has a broad set of social features.

The app's search function and one-way money transactions make it relatively easy to track down a celebrity's account and send a payment along with a comment of appreciation. The amounts transferred are often just pennies or dollars, but the transactions muddy Venmo's intended use and raise some questions of propriety.

Prian's payments to Trubisky, for example, fall into the questionable territory of gambling and pay for performance, according to Jeremy Evans, managing attorney at law firm California Sports Lawyer.

"Sending money through Venmo for gambling purposes is most likely illegal on a state-by-state basis and also violates Venmo's terms of service," Evans told CNBC in a statement. "We can take that a step further and say that such payment for performance, if not necessarily gambling, most certainly violates the regulations of the various players associations and professional sports leagues as to impropriety and pay for performance schemes."

In other words, it's not exactly like betting on a game, which is against league rules for players, but it's murky since the payments are tied to a player's performance. Another argument is that the payments constitute a gift, "but that still has legal and regulatory implications," Evans said.

Venmo's terms of service explicitly prohibit sports betting

Prian said the payments to Trubisky were mostly a joke. On top of the $8 he paid the quarterback for a win, Prian said he tacked on an extra $8 whenever backup QB Chase Daniels stepped in, so Trubisky could cover a second beer for Daniels.

"The joke just kind of kept going on," said Prian. He sent more than $80 dollars in the last year and said the money is "100 percent worth it for the joke and for the potential for him to know who I am."