In 2009, there were more than 20,000 attacks that peaked above one Gbps of traffic. Arbor Networks looked back at the data collected by about 100 of their ISP customers on DDoS attacks in 2009. The probability of an effectively-sized attack targeting a given Internet property today is higher than the probability of a fire that affects that enterprise s Internet availability and online presence (something I ll look to qualify) whilst from a business continuity perspective the latter is quite likely what the enterprise values most in the connected world.
Source: https://threatpost.com/large-ddos-attacks-still-serious-problem-011110/73351/