If I were in charge of marketing for a formula company I would totally have people posting on MN, especially as many mainstream marketing avenues are so restricted.
I think you can be a lot more subtle than just pro-formula/anti-breastfeeding posts, though.
For instance: An OP posts, "My EBF 6 week old baby won't stop crying" Then there are a few avenues you can take:
"Are you sure baby is getting enough milk?" (Undermines breastfeeding confidence, but won't arouse suspicion as it's a genuine possibility)
"Get your DH to give some formula so you can have a rest. You can go back to breastfeeding refreshed" (A very kind suggestion and formula top-ups/mix feeding are obviously perfectly possible, but if not done in the right way, there's a risk of diminishing supply and an early end to breastfeeding)
"Maybe it's something you're eating? Try cutting out dairy/ chocolate/ curry/ gluten/ eggs/ brassicas, etc. (A rare but real issue, makes breastfeeding look much harder for the mother and helps support the myth that only people with perfect diets should breastfeed)
"Maybe it's silent reflux? I switched my lo to (formula brand) and she was so much happier. (Again, plenty of babies diagnosed with silent reflux so it's a reasonable suggestion, but because it's "silent" it would be easy for a parent to misdiagnose, and a high rate of misdiagnosis would certainly improve my hypothetical sales of a formula, especially if I get my team to all recommended the same type of formula)
You don't even need feeding type mentioned in the post. You can respond to any baby problem with "Are you breastfeeding? Maybe it's x" Even if the OP isn't breastfeeding, lurkers might be.
And on the crying posts, don't mention that it's normal for babies to cry, with a peak at 6-8 weeks. Because if a Mum can have her confidence in BF undermined at 6-8 weeks, she might switch to formula and then her baby will cry less and sleep more. They almost certainly would have regardless, but some (not all!) of those mother's will think the change is due to the formula and continue to advise other mothers, thus doing my marketing team's job for me.
Now, I don't actually work in marketing, but I wonder if this approach has to be worth a few salaries out of the massive marketing budget.