What percentage of the population has been exposed to the Epstein-Barr
virus? Not people who actually developed a case, but just have been
exposed?

Hello yesmam,

The answer to your question is pretty well everyone. 

I had the privilege of studying for my first degree and PhD in the
Department of Pathology in Bristol University, UK in the days when
Tony Epstein, the Epstein in EBV, was the head of department.  As you
can imagine, we studied the patterns of EBV infection a lot.

The key to the outcome of infection with EBV in Western societies is
the age at which it occurs.  Most people are infected in childhood,
and show no specific symptoms, maybe just a runny nose and flu-like
symptoms for a few days.  After that, they become immune to any
further infections.  Those who escape infection until they are young
adults are the ones who are most likely to develop infectious
mononucleosis (IM, glandular fever) as a result.  IM is often called
the "kissing disease", because it is spread so easily through kissing
among teenagers and young adults.
http://www.kcom.edu/faculty/chamberlain/Website/lectures/lecture/mono.htm

Here are some further views:

"Infection with EBV occurs worldwide among humans and usually occurs
as a subclinical infection in early childhood. About 70% of the people
in the U.S. are infected by 30 years of age. Infectious mononucleosis
occurs in young adults 15-25 years of age. EBV is acquired by contact
with infected cervical and oral secretions (kissing, sharing drinking
glasses, etc.); EBV can be isolated from saliva. EBV can be
transmitted via blood transfusions. Several doses are required for
infection since EBV is not very contagious. The virus is still present
in the saliva for months after the patient recovers. More than 90% of
EBV-infected people intermittently shed virus for life even when
asymptomatic."  http://www.kcom.edu/faculty/chamberlain/Website/lectures/lecture/mono.htm

The pattern is quite different in other parts of the world.  For
example, in the malaria areas of Africa, EBV can cause the development
of a cancer known as Burkitt's Lymphoma.  In Southern China, EBV is
considered to be a causal agent of nasopharyngeal carcinoma (cancer of
the back of the nose and top of the mouth).

When I was a student, the people doing research on EBV in my
department were desperately keen to get hold of blood serum from
people who had never been infected, and they were also delighted to
get serum from people in the active stages of IM.  One of our
laboratory practicals was testing your own blood for antibodies to EBV
(if proof that you had been infected).  Knowing what might happen if
they found my blood was negative (ie men with large syringes and
measuring cylinders knocking at your door), I made my excuses and
left!

Please use the clarification facility if there is anything which I
have not clarified sufficiently.

Wow! You are great. Thanks.