European Competition Commissioner Margrethe Vestager speaks during a news conference at the EU Commission headquarters in Brussels, April 15, 2015. Francois Lenoir | Reuters

Europe has made its name as the top cop of tech regulation. The European Commission, the executive arm of the European Union, has imposed a combined $9.5 billion in antitrust fines against Google since 2017, and its boss hints Amazon and Apple might be next in line. Facebook, meanwhile, has been subject to probes from competition and data protection authorities across the EU since the region's strict new set of privacy rules called the General Data Protection Regulation (GDPR) went into effect last year. As U.S. regulators and lawmakers step up their efforts to reign in big tech companies, Europe offers some valuable lessons. "You've seen a lot of proxy wars go on here because of the nature of the fact that we've got one Europe-wide regulator for antitrust, and the laws are clear, and that regulator definitely has teeth," Mark MacGann, Uber's former head of public policy in Europe and founder of venture capital and advisory firm Moonshot Ventures, told CNBC in an interview.

When it comes to enforcement, the European Commission has opted for fines and mandates for tech companies to their change in business practices, rather than arguing to break them up, said John Cassels, a partner who specializes in competition law at European firm Fieldfisher, in an interview. This tactic could put Europe's approach at odds with some U.S. lawmakers touting strong rhetoric about dismantling big tech companies. The EU's fines have done little to dent any tech companies' profits. "The Commission looked like it was going to go as far as some sort of break-up but actually it's sort of shied away from that recently," Cassels said. Here's how Google, Facebook, Amazon and Apple have fared among EU regulators so far.

Google

Alphabet subsidiary Google has faced the toughest antitrust action. In March, the European Commission levied a $1.7 billion fine on the tech giant for abusing its dominance in the online advertising market, citing "illegal" practices of restricting competitors' ads in search results. Google said this week it is appealing.

The fine marked the third consecutive year of antitrust rulings on Google from the EU. In 2018, the regulator charged Google with a record $5.1 billion fine for anti-competitive practices on its Android devices, saying the company had forced device makers to pre-install its own apps. Analysts at Evercore ISI wrote in note last week they expect the U.S. Department of Justice could take cues from this specific case, which forced Google to offer browser and search-app alternatives on Android devices. "We believe this could be an area where US investigators take a much closer look then previously, given the now far greater importance of the mobile ecosystem vs 2011," the Evercore analysts wrote. The European Commission first went after Google in 2017 with a $2.7 billion fine for abusing its dominance as a search engine. The EU accused Google of favoring its own comparison shopping service over competitors' in search results.

Legal experts said the 2017 findings, which were released after a nearly seven-year investigation, could also serve as a guide for antitrust action in the U.S. when it comes to Google's search practices. But they also cautioned it's easier to prove market dominance under EU legal standards than in the U.S. "In the U.S. the thresholds are higher and the FTC and DOJ are sort of put to the test by courts," Fieldfisher's Cassels said.

Facebook

Facebook's EU headquarters is in Ireland, and the social media giant is facing several inquiries from the Data Protection Commission there. The complaints range from Facebook failing to provide access to personal data to how the company targets ads on its platform. "You're going to see a lot of decisions over the coming months by national regulators in Europe especially in Ireland," MacGann said.

Experts also point to the findings of a three-year long investigation of Facebook by Germany's antitrust watchdog, which could serve as a guide for a possible FTC investigation because it tackles both data collection and monopoly concerns. In February, the German regulator found Facebook had abused its market dominance in the way it collects, merges and uses user data across its platforms, including WhatsApp and Instagram. It ordered the company to stop merging data on separate apps without users' deliberate consent. Facebook appealed the order.

Amazon

Germany's antitrust watchdog is also investigating Amazon. At stake is whether the retail giant prevents fair competition in its online marketplace in the country. The regulator cited "numerous complaints" it had received from sellers about Amazon's business practices. Amazon's role in e-commerce has prompted recent probes by national competition authorities in Austria and Italy, too. In the case of Italy, regulators singled out Amazon's practices in both online retailing and logistics. Amazon has said it is fully cooperating with authorities. Meanwhile Margrethe Vestager, the head of competition policy in Europe, told CNBC in April the EU's investigation of Amazon is in an "advanced" stage. The European Commission is examining whether Amazon is taking advantage of merchants' data.

Apple