In another thread started at the end of 2007:
QUOTE(Brandt)
What is the life expectancy of Wikipedia?
...
I give it three more years.
QUOTE(GlassBeadGame)
Two and a half, tops
QUOTE(Text)
Percent of global Internet users who visit X
Wikipedia: 13%
Youtube: 23%
Facebook: 33%
Gogl: 43%
After a strong force obscures wikipedia, it will fall down around 3% reach in 3 years (similarly to Myspace 2007-2010) and around position 20 on alexa
That was speculation made between 2007 and 2010.
It's 2013 now. Alexa reports that this year, Wikipedia went from sixth to seventh or eighth most visited site in the world, being surpassed by Amazon and Baidu (first in China, fifth in the world).
Facebook, Google, and Youtube are the top three sites. Myspace is sharply falling down, currently being the 411th most visited site.