You can offer more insights as to why you found the comment to be of value and this is really important for Facebook to learn what made you react in the way you did!
The main question on all hosts and community managers lips right now is “will it work?” and in truth it is going to work as well as any of the features on social media. It is user specific which means that of course there will be some users who accept, adopt and use the function correctly but then there will be others who abuse it. It will be a battle of wills in terms of opinion over “what is valuable” but it does allow for popular and more useful comments to have the potential to be raised to a higher visibility.
For those of us who are commenting and supporting as a visibility metric and audience building practise it does give us the opportunity to be “rewarded” for our valuable support which is often given and lost in the ether. The polar opposite to the author “pinning” their favourite comment to the top over on Instagram, this is very much community based and does not rely on the author alone to pick a favourite – thus giving a truer indication (when used correctly) as to what has been organically well accepted by the community.