A single word for an "instance" of publicity

Opportunity. It's been very widely used for years in the phrase "photo opportunity", often abbreviated as "photo op" or "photo opp", but Ngrams shows a dramatic increase in uses of the phrase media opportunity, which I think is what you're looking for.

In Australia, the word 'promo' is quite common for indicating a piece of advertising material; e.g., a promotional article, review, TV segment, or whatever the case happens to be.

Another widely used colloquial term is 'plug'...

Here's the definition from thefreedictionary:

Informal. A favorable public mention of a commercial product, business, or performance, especially when broadcast.

And an example:

"If you help me out here, I'll slip in a free plug for you next time I'm on air."

It's not exactly what you asked for, but why use the singular? You could rewrite it as "Each of those articles or reviews are examples of publicity for the company."

a mention

You got a mention in the paper the other day.