"I don't know how you could turn that offer down, honestly," he said. "I'm shocked that somebody would turn that kind of offer down."
The video messaging service popular with teens—and the Wall Street crowd—would help Facebook reach the elusive adolescent demographic, Mahaney said.
(Read more: Wall Street into Snapchat, and regulators are on alert)
"Sure, why not buy Snapchat for $3 billion. Makes sense to me, actually," he said. "I think Facebook stock would've traded up if they'd announced that deal."
Mahaney, whose top three tech picks are Amazon, Priceline and Netflix, also weighed in on two other names in the sector.
Yahoo: "This company has fundamentally underperformed the entire industry for five straight years. I think it's going to be six before this year is done," he said, noting that many investors were buying it for its stake in Alibaba. "That trade can still work for a little bit longer, but I don't think there's that much upside left."
(Read more: 'No way' Twitter's worth what it's trading for: Pro)
Google: "It's one of our top picks," Mahaney said, noting that the company likely had good days ahead. "Two to three years down, I bet you this'll be the single best derivative play off of wearable devices, wearable technology. So, it's got a nice core search business, it's got this huge, great asset in YouTube two, and it's got an option play two or three years down the road."
—By CNBC's Bruno J. Navarro. Follow him on Twitter