Jehovah's Witnesses have enjoyed impressive growth during much of their history. They are still one of the fastest growing religions in many areas. However, their rate of growth is slowing down and they are facing a growing trend of people leaving, reducing their Witness activities, or being kicked out (either for committing some sin on the long list of Jehovah's Witness prohibitions or for apostasy, which basically involves disagreeing with the Governing Body of Jehovah's Witnesses, even if they don't actively promote their doubts). It is getting progressively harder for them to replace the members lost through attrition.
They are facing other new challenges. The Internet has advertised their comical history of false prophecies and doctrinal flip flops. The quotes from their own literature can be easily found by anyone with an Internet connection. (See this quotes site as an example.) Most of their remaining growth is now coming from immigrants. This means that the people coming in are often less able to contribute financially, while still using up publications and other resources. Many experienced witnesses are now moving into foreign language congregations to help serve their needs. This is making it harder for the more established congregations to find people to serve as elders or otherwise take care of the congregation responsibilities. It doesn't seem that the younger ones are nearly as eager to do this as they were in the past.
It used to be that the Jehovah's Witness strict expulsion and shunning policies helped to enforce unity. Now, a growing number of members have learned the truth about the organisation and are only staying in to avoid losing contact with family and friends. These are some very disgruntled people who desperately want to leave, and probably will if they can convince enough of their friends and family to come with them. In the meantime many of them are trying to use subtle means to undermine the organisation's authority. This is a tremendous source of potential instability among Jehovah's Witnesses.
It is my opinion that the Jehovah's Witnesses rate of growth will soon drop below the growth of population increase and may even stagnate at zero or become negative. I don't believe that the Jehovah's Witness mindset can long tolerate stagnation. Their worship is based on converting people before the end. I think some sort of catastrophic collapse is a plausible outcome of what is now taking place.
That's just my opinion and I'm sure others disagree. I admit that my perspective may be biased by my concern for friends and family that are still stuck in the cult. I recommend this discussion at Jehovah's-Witness.net. It discusses a British Sociologist's prediction that there may be a collapse in Jehovah's Witness membership and has responses from people who both agree and disagree with that assessment.