Is it illegal to advertise via spam?

[Kim]

Is it legal to promote your business on social media via spam?

Of course, it may be against the policy of the social media site to spam, but can you get sued for it? What if, hypothetically, I spam yahoo answers to get people to notice my business. I know that the owners of yahoo have better things to do than sue me, but could they technically?

Also, I'm aware that spam gets deleted and that it's against the terms and conditions of nearly every website. I'm just wondering if I could be sued for it.

What if, hypothetically, spamming on a website does attract a substantial number of clients, and a business thrives? Are they then more open to being sued since they profited from spamming?

FYI, I am thinking about doing this, but not on this website. I understand it's annoying, but I'm hoping answers will focus on the legality of spam rather than their distain for it.
@ Inda Cloud

Thanks. Do you happen to know whether what you said also applies to Canada? I think I should have specified.
@Listener Sheogorath

I don't see how that makes sense. Think of what you're saying and it's implications. If I sent "gay propaganda" to someone in Sochi, where it is illegal, there's absolutely no way in hell I'd be prosecuted in Canada for that If it's not illegal here they won't have any success in suing me =P