You wonder how web sites like FB know about email addresses you maintain without being explicitly told. I can't tell you how they got hold of your Yahoo address, but it must have been from a publicly accessible source. Despite what you claim as certain, it is most assuredly not because they are somehow hacking your Mac.
Beyond the obvious (you or your friends somehow publishing your info like DK described) there are various ways in which you as internet surfer stand out. Some of these details are discussed in the thread
Web privacy: cookies vs browser specs. Related privacy/security concerns were discussed in several MacWorld articles I linked to in
THE CYBER-SECURITY THREAD.
Another tool I can recommend in this context is Safari extension
Ghostery, which shows you who's tracking you when you visit any particular web page. Install it, visit these forums again and see for yourself. Then compare that with other sites you frequent.
As to whether FTM can access your cookies, I can say that as a mod I can't with the UBB.threads tools I have access to. It's possible that admins have access to such tools, but to be sure they would have to chime in with the details. That said and assuming they have such access (it would be a UBB software-based privilege), I'm sure that they aren't likely to use it if only because they have better things to do. That may not reassure you, but it's good enough for me. Btw, the FTM cookies control your access to the forums, nothing more, nothing less.
Finally, the email address you need to provide FTMF upon registering is required so admins have a way to contact you directly, in case contact via the forums is not possible or desirable. Even for that purpose we don't use these addresses lightly, nor (AFAIK) do we provide 3rd party access to them.
