I'm the OP on two long running threads on S&B, I've been offered free stuff by companies twice over the past few years, only once on the back of those threads. So IME it doesn't happen often but it does happen.
For disclosure, one was from someone starting a company who was also a MN poster who I helped out with their branding and one was a company I recommend and they wanted to reward me for mentioning them to someone else on one of the long running threads. I refused one as it wasn't something I was interested in and made sure the 'reward' recommendation went to the person I actually recommended it to rather than me.
I'm not an interesting 'sales poster' though, tend to dissuade people from overpriced skincare and keeping it simple on one thread and the other thread is more concerned with small niche European brands and designer clothing and mindful shopping. They're not the ones interested in the traditional MN demographic in general.
I am always quite suspicious of instant MN faves though. Currently it seems to be Lucy and Yak where the MN s&b boards have traditionally been a bit sniffy about dungarees and branding it as adult toddler wear while clutching some pearls. Saints and Sofia also had a go (and continues to try to flood a tear down thread with positive reviews whenever it resurfaces, but they haven't been very successful which is probably quite frustrating as MN threads will often show up high on Google searches for their demographic).
But don't underestimate the power of Facebook and Instagram ads and influencers that target the MN demographic. There's a lot of overlap, that's very visible here but looks suspicious if you're not on those platforms too or are not that traditional demographic. I believe for years the Hush threads (not sure if they're still ongoing) were treated with great suspicion, the Hush PR
department has shown up on other threads before and MNHQ often recommended them in their newsletter thingy and offered special codes IIRC. It definitely was part organic, and the majority of the posters on that thread were genuine but it would be naive to say the company wasn't at the very least monitoring and taking notes and potentially posting on there themselves.
Official term is dark social and it happens everywhere, going so far as private WhatsApp friend groups, MN probably sees as much of it as anywhere else although doesn't seem very aware of it themselves as we can only report very obvious spam and anyone with a posting history is given the benefit of the doubt even when it reads like very obvious spam.