Networks:
Economies of scale mean only a few cloud service providers dominate the industry. The three top companies, Amazon Web Services, Microsoft Azure, and Google Cloud, control two thirds of the market between them, concentrating outage risk.
Websites:
Websites are natural industry aggregators. Common interest groups gathering on these sites, or subscribing to industry-specific blogs, are the targets for watering hole attacks.
Software:
Operating systems are probably the greatest risk aggregators. Counting both cell phones and computers, the seven billion devices in use today use only three types of operating systems: Microsoft, Apple OS, and Android. But industry-specific enterprise software for, say, dentists can also be dominant in specialist niches.