Instead of FB, etc getting ad money, that ad money is being spread out to many others instead. It also seems like this might be more cost effective than ads.
I think I would consider this much more ethical than how Google puts the top 3-5 results as "ads" these days. Sometimes I question how long until I have to go to Page 2 to get off the advertised results.
Note - In the above I'm only considering this practice being done by legitimate places sharing fairly accurate or honest content. Such as a business trying to promote a sale or a blog piece.
I would consider paid ads, paid likes, paid comments, etc., to be unethical if the content they're supporting is false.
TLDR: What's the difference between paying Facebook/Google/etc for an ad vs paying someone else to like your post? The 2nd seems like a better solution for a majority of people.