Receiving multiple transactions with one address is fine until you spend with it.
Not really. If I hand out the same receiving address to my employer to pay my salary and a co-worker who owes me $5 for lunch, then my coworker can now see how much I get paid.
According to the context of his post, i assume he was talking about the security aspect in which case it indeed doesn't matter.
Privacy-wise you are right. But the average person with a facebook account doesn't care about privacy at all and therefore probably will use the same address for everything.