Americans probably know Dear Abby is a real person. I didn't, and I was glad to find the following article (also some bits from John Prine himself):

[1993:] I was in Europe, and my first wife and I stopped in Rome for the day. I wanted a newspaper, and all they had was the International Herald Tribune, which is all the tragic news in the world crammed into six pages with no sports results and no comics. And yet here's 'Dear Abby'. She was the only relief in the whole paper. And that's where I wrote most of the song - in Rome, Italy, that is. Years later, somebody took the verse about the guy whose stomach makes noises, wrote it just out of kilter enough so it didn't rhyme, and sent it to "Dear Abby." And she answered it in her column. She suggested that he seek professional help. She got loads of letters from people who knew the song and told her she'd been had. (Notes 'Great Days - The John Prine Anthology')

[1997:] In January 1956, a sassy dame called Pauline Phillips telephoned the editor of the 'San Francisco Chronicle' and told him she could do a better agony column than the one he published. He invited her to call in and found himself facing a middle-aged housewife who had never written professionally but claimed she could write an advice column because all her life people had confided in her.
Now there is a standard drill for dealing with this kind of person - you ask her to write some sample columns. This usually fixes them. But not Mrs Phillips, who wrote some sample material under the pen-name 'Abigail Van Buren' and called the column 'Dear Abby'. Within a few months, it was syndicated across the United States. Now it's on the Net [...] and readers all over the world can see what has kept millions of American readers laughing for decades. [...]
'Dear Abby,' writes Exhausted in Honolulu, 'I've been married for five months to a 58-year-old sex fiend. If this letter makes no sense, it's because I no longer know what a decent night's sleep is. This man is an absolute machine. His demands are exhausting!' Abby advised Exhausted to tell her husband to see a doctor or find a woman who was more his speed, but letters poured in from other readers offering to help out. [...]
Most journalists think that agony columns are money for old rope because the readers write them. Not true. The readers provide the opener, sure, but it's the aunt who has to deliver the punch line. It is her skill at doing this which makes Abby the best thing since Dorothy Parker. 'Dear Abby', writes Gertie, 'I've been going steady with this man for six years. We see each other every night. He says he loves me, and I know I love him, but he never mentions marriage. Do you think he's going out with me just for what he can get?' 'I don't know', replies Abby, 'What's he actually getting?'
Or how about this from 'Annie'? 'Dear Abby, I know boys will be boys, but my "boy" is 73 and he's still chasing women. Any suggestions?' 'Dear Annie', comes the reply, 'don't worry. My dog has been chasing cars for years, but if he ever caught one, he wouldn't know what to do with it.' (John Naughton, Observer, 8 June)