With respect to clever uses of participation that used as a marketing strategy by some companies, I did witnessed some successful tricks that actually works for finding potential customers, and even driving them to ask for the product actively.
For instance, some small companies will run their facebook, twitter or instagram accounts by posting their promotions positively and always show their best attitudes with trending hashtags so as to increase their exposure rate to as more people as possible.
Another example is when people are reading posts on some very popular forum websites like reddit and github, there will always be some susceptible “soft advertisements” that looks very familiar to a usual post but has implications that some products are actually really good. But in fact, those posts are very likely to be traced back to the companies who sells the referred products.