Complex network is vulnerable because it's strong interconnectivity. It might be one of the biggest characteristic and one of its best features; however, it becomes a vulnerable point where the network can be easily attacked. Robustness means how a cell survives and function under different external conditions and internal errors. In order to achieve robustness, it is essential to learn from interconnectivity. The tolerance shown in a cell is the range of error it processes. Basically, under the theory of topological robustness, removing only a few nodes will do very little impact on the whole network. Only if the number of removed nodes reaches certain critical point the system would start corrupting.
Basically if we think like a terrorist, we would understand that there's no point of having 100 anonymous citizens as hostage to negotiate any deal with the government. The best way to get the police or the SWAT or any other weird force team that you see in movies to get you a plane from JFK to Hawaii would be threatening someone important. We can see that someone important as the hub. (In a sense, the important person is a hub because he/she exists in the middle of complex social network. There would probably be malfunction of certain organization due to the absence or death of this important person.) However, let's think more about those 100 anonymous citizens. Terrorists still pick anonymous citizens as hostage more often than important people because they are more accessible. There are so many random people walking around on the street but there's only one George Bush. (Personal Note: NOT trying to imply that terrorist would be against Bush.) Small nodes are more likely to be dismantled because there are many more of them. We see this master plan of hostage as a system of scale-free network. If 20,000 people were killed in NYC it would be a problem, which is the example of 911. (20,000 as a hypothetical number because I don't want to be too political correct about tragedies.)
A scale-free network is not vulnerable to failures; however, it is very fragile under attack. A terrorist can kill all the people on the block but it won't destroy the democracy system. However, when the twin tower is bombed, it does impact the whole economy in a ripple effect. If we remove air and water, nothing will survive. If we remove water from the world, we might still be able to survive. The assumption is that several of the largest hubs must be simultaneously removed to crush the system.
