Here sharing is different from promoting. When we share it will shown to our timeline as shared post. But when we promote it will be frequently posted to friends wall as a featured post. It is said that it will be according to privacy settings. When we share, we don’t see who originally posted, but when we promote it will show who promoted giving credit for promoting.
We have to pay Facebook for promoting, it says $7 is common fee. It is another plan of Facebook to increase it’s revenue.