I do not think it has a big impact, but the 'ROI' is obvious if only because the cost is so low. Social media can be done very inexpensively. If a manufacturer has already done a case study or issued a press release the incremental effort to post it on social media is trivial.
Based on our own social media stats, there's not a ton of industry people reading twitter daily. An average IPVM tweet gets viewed ~500 times, according to Twitter analytics (and we have more followers than the average manufacturer.
Of those ~500 views, most are certainly existing IPVM members and overwhelmingly manufacturers and integrators (who seem most motivated to follow such things daily). I suspect the pattern is similar for manufacturers.
Because of that, it can be useful for keeping existing customers informed ("Look, my partner Pelco just did a school near us", etc.) but I think it rarely has sales impact.
For a bigger impact, do social media advertising.
What I would recommend is Twitter and LinkedIn advertising, which can be very finely targeted to expose one's offerings to new customers (e.g., see: Manufacturers, How LinkedIn Ads Are Working For Us).
Here's a neat tactic to try out if you are going after an enterprise account. LinkedIn lets you target by a company AND role (e.g., you can restrict an ad to only be displayed to security managers at Disney World). if you are trying to get into a major account or opening up a new market (e.g, cruise ships), you can use LinkedIn to increase awareness to just those specific people you want to reach. It makes them take notice of you, gives the impression that you are big and can be done very inexpensively.