Speaking with CNBC's "Squawk Box," Mellon said the days of rapid growth for the companies are behind them as increased government regulation looks more likely.
"These fatted calves are now ripe for the plucking by governments everywhere," he said, adding that tech companies were going to be "pillaged" by administrations across the world.
Drawing a comparison to the increased scrutiny faced by banks following the 2008 global financial crisis, Burnbrae Group's Mellon said the fines are going to be "enormous" and will likely be "bigger than those being levied on the banks in the last decade or so."
Turning his attention to Facebook, Mellon said: "If I was Mr. Zuckerberg, I'd be looking at a halving of my share price in the next couple of years, basically."
"The Cambridge Analytica thing is just the tip of the iceberg, in my opinion," Mellon said, referring to the ongoing scandal related to the political analytics firm's use of Facebook user data.
"We're going to see decimation of particularly Facebook," he added, saying that it was a "trivial use of modern technology and one that's rather sinister."
Mellon has been historically bearish on big tech stocks and is no stranger to controversy. In 2016, following the U.K.'s historic decision to leave the European Union, he told CNBC that "the euro is gone within three to five years, as it currently exists."