When you visit Wikipedia, it is claimed that there are over 1.5 billion users. Furthermore, it is estimated that a quarter of a million people join the network each day.
Facebook was started in 2004 by Harvard student Mark Zuckerberg. It reached its first 100 million users milestone in August 2008, and the exponential growth has been unique to an extent that even Microsoft expressed an interest in buying. Facebook was, however, put onto the stock market around 2012, and subsequently, one can only imagine that user numbers could be inflated so as to appear of greater value towards potential investors.
There is one obvious limit to the number of users, and that is based upon the total number of computer users, which is one logical limit. Another limit is based upon the changing Terms of Service which have caused quite a few not to register on that network due to skepticism towards personal privacy.
We should however realize that search engines have a long tradition of surfing the web, so nothing really stays private anymore, unless you put in technical hindrances, and even then you can’t really be certain that all search engines will respect such limitations.
When analyzing social networks one cannot avoid mentioning Google+ and Twitter. Google+ has a far smaller user audience, and even that is challenged by the fact that you are often given a profile on Google+ when you register for an e-mail account at Gmail. One common aspect between Google+ and Twitter is that they are known as common interest networks, so user involvement is often much higher even though the user base is somewhat smaller.
Most people involved with social media would therefore recommend that you use more than one network so that you can spread your activities so you can reach people whatever personal preferences they may have. I would agree with that, since there are fields of interest that fit naturally into either Google+, Twitter or even Instagram. Others have a preference for Pinterest, and consequently you need to check out each of these networks so you can use whatever fits best for you.
Whether Facebook will continue its exponential growth depends on a lot of factors. Especially whether they can come to grips with some of the hate groups and hateful rhetoric that is rampant in some areas of that network. You can easily lose people when you have timelines filled with ads combined with comments that are less than polite, and that is the biggest challenge for Facebook.