Thanks for the great question.
As someone who has an interest in a publishing business (including digital/online publishing) I can share with you our experiences since I got involved and started shifting the business from print to a digital-centric approach.
The first thing we did was to completely rebuild one of our key websites, which had a lot of great content but poor social features. We combined this with preparing our social media presence on LinkedIn, Facebook, Twitter, and Google+.
We then actively engaged with our contributors (writers of articles from our magazine, which are now published online) and invited their feedback and participation.
It's important to understand that, for us, eyeballs on website and newsletter subscriptions translates to an audience that advertisers will pay to reach. Our magazines are for vertical markets (education and security) and we have something very specialised to offer those audiences.
To put it in perspective, in the first month of launching the new website we have doubled the number of visitors to the site. This was achieved in large part by social media. We spent a bit on Facebook and Twitter advertising, but the lions share of the visitors have come from social media engagement.
As one example of this, we posted a single article and the author of the article shared it amongst his networks. We had over 600 people read that single article in just the first 48 hours after publishing it. About 20% of the viewers came from our own direct sharing, and 80% came from other people sharing it on our behalf. The author's own networks were the largest part of that, and it was all free.
If I were a small business trying to use social media to attract new clients, and I only measured success by the number of paying clients I attracted, it could be a much tougher proposition - not because social media doesn't work, but because we frequently don't engage enough with other influencers in the social networks, and we rarely have enough exciting things to share and talk about.
Having worked in digital marketing for 10+ years, I know that social media is not an automatic winner - it requires a lot of effort, creativity, engagement with other people, and you need to find a real point of difference, something valuable and engaging to share. But when you master the art of doing this, social media can be amazing.
Just to throw in one final example - I have a friend I met through professional networks who runs a Virtual Assistant business. She started off alone and now sub-contracts something in the order of 20+ additional people to do work on her behalf (casual, not full-time). She's making a good six-figure income out of it, and I recall her saying back in 2014 that she'd attracted over 200 clients directly through social media. However, she's running out of energy to continue being so proactive in social media and the nature of the beast is changing (particularly Twitter) - however, she's fortunate enough to have such a large client base now that she's always got repeat business and word-of-mouth referrals coming in and is no longer concerned about using social media to the same extent.
Had to put my two cents, since I know Jeanette is very up with the online world. The experience I have had with SMM is: a) it's a daily slog in building an audience even with my creative talents, only brightened by sharing 'real tips' in the friendlier groups, ROI - jury still out, and b) for online campaigns it needs a budget, just like any other medium, to get more leverage. Unless you really want to while away your time.