Hi Laurence -
Now that's an interesting question as it raises the issue of myself needing to improve on G+ as I am sure there is much more to this platform than meets the eye...
That said my money is on FB - thats because of the sheer numbers of users and groups which you can connect to and consequently leveraging that for traffic to your site... In addition FB ads has some excellent targeting capabilities based on age, gender, location, hobbies, interests, employment etc...
I've found direct sales does not work on social media - and I can understand that as its annoying to see such ads as well as the fact people are on these platforms to socialize and not buy things... Hence for me the indirect approach is best such with capturing leads and in turn making a soft sale once I have developed a relationship.... BTW - One of the best social media campaigns that I have launched relates to quizzes such as those buzzfeed ones with at the end of it recommending the user to sign-up for more info on a specific topic... Memes, Infographs and of course informative, helpful articles are other methods too... In otherwords I would focus on fun and engaging means to reach out as that can go viral and bring in more organic traffic through shares etc...
I very much hope that helps....
On that note I'm really interested to hear of other views... especially on how to improve on G+ ....
Best Regards,
Sid