I would say the first question is easier since it pretty much states the definition of a defamation since people stopping to ridicule him obviously lowers his standing amongst right thinking members of society generally and it states he's lost business.
A case specifically relating to social media you could use is Lord McAlpine v Bercow which you probably remember from the news is where Sally Bercow was sued for her tweet which said 'why is Lord McAlpine trending *innocent face*' after the newsnight documentary which talked about an unnamed senior tory MP involved in sexual abuse. The case is mainly about innuendos but you could mention it for the social media element I suppose.
Now I think about it it might be worth mentioning, if you feel like you don't have enough to talk about, that he could potentially bring a claim against the social media site which could (though not likely) be liable as a publisher of the material and there are some relevant cases on the matter such as Tamiz v Google.
Defences are what the defendant, either B or the social media site, could try and use to rebut a claim. There's numerous defences for defamation such as truth, honest opinion, privilege and innocent dissemination some of which definitely seem to be relevant in this scenario.
Defamation is typically regarded as a strange and unfair tort with a difficult balancing act between individual's rights on one hand and Article 10 ECHR right to free speech on the other which is why they passed a new defamation act which came into force on the 1st January this year. After Ian Hislop had to pay £600,000 damages for defaming the Yorkshire ripper's wife he said "if that's justice then I'm a banana" which pretty much sums it up.
Interesting that you actually know what question's you'll be asked on your exam, wish it was the same for me.