It seems that there are many contradictory opinions as to whether it's advised or not to put social sharing buttons on a landing page.
Some say it's good and some say it lowers the conversion rate.
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If you want to increase conversion rate on a web page, landing page or not, social sharing is important. This is because users who follow/are friends with each other share an interest, a school, a workplace, a group or any other social relationship and because of that are more likely to use similar services and products.
Remember also that social sharing is more trusted among users than general advertising. Therefore social sharing should be present where you find products and services.
Social Sharing: Do not forget to give visitors an opportunity to share this amazing offer you are giving them. There is a marketing principle that states people who have similar interests and fit in similar profiles typically hang out together. There’s a good chance that the visitors friends, family or coworkers could also be great targeted prospects for your offer.
Leaving landing pages behind, there are other sources on using social sharing to increase conversion rates:
THETFORD CENTER: Digital marketers regard social sharing and third-party credibility indicators as two of the most effective ways to boost conversion rates, research from Adobe, the software company, has found.
The Adobe 2013 Digital Marketing Optimization Survey, which received global responses from more than 1,800 digital marketers across North America, Europe, and Asia, asked marketers to rate the effectiveness of various digital elements in increasing conversion rates.
It found that 39% saw activities such as putting social sharing icons on pages to be "very effective" while 37% indicated that third-party credibility indicators, such as security seals, achieved a similar outcome.
Reference: Social sharing boosts conversion rates
Still, that may not convince you, but I wouldn't leave social sharing out of any web page. The best would be to start measuring before and after that kind of change to a web page and measure the outcome. It's the only way to be sure that your improvement have the desired effect.
If you're still in doubt, take a look at professor Eric Qualmans video Social Media 2013.