There is strength in numbers. If that is true, then ISIS is strengthening greatly. Unfortunately, their numbers are growing by leaps and bounds.
Earlier in the summer, they were thought to have 10,000 fighters. Then, the CIA said that, by August, they had grown to a force of between 20,000 and 31,500. Now, it is estimated they have a militia strength of between 30,000 and 50,000 and, with some experts, outside of our government, saying the number is as high as 100,000.
Obviously, the coalition airstrikes and the fighting on the ground by the Kurds, the Iraqis, and the free-Syria rebels isn't working. Otherwise, ISIS wouldn't be growing so rapidly. If this growth rate doesn't abate, they will literally be able to overrun any of the current ground forces that are fighting them in Syria and Iraq.
Is ISIS Growing? Feinstein said Sunday on CNN that "ISIS is essentially a fighting force of 30,000 to 50,000 people.": http://thehill.com/policy/defense/221949-is-isis-growing