Idiens, Google and Yahoo may track things--but they don't send spam. They target the ADS which they display, but they don't send spam, and anything they do send shows them as the "FROM" address, they don't harvest and sell your directly for spam.
On the other hand--any email address which is PUBLISHED openly on any web forum, including the moderated Yahoo groups, will be harvested and sold by folks who get paid per address.
The bottom
line is that you cannot "publish" your email address anywhere on the web, unless it is a disposable address and you change it every couple of months to ditch the spammers.
I wouldn't be surprised if some enterprising folks in a 3rd world country have hired rows of hungry folks and said "OK, you join this forum, then look at the memberhsip list, and copy every email address shown for every member. We'll give you two cents per hundred addresses." Because in some parts of the world, that IS a profitable business, and a dollar a day is a great wage for indoor labor.