I think you are
I can hold the heaviest of all objects
Diamonds, made of carbon, are the hardest known material (though some people only say they are the the hardest naturally occurring material, and there seems to be some controversy as to whether they are in fact the hardest), so a large diamond would presumably be capable of holding very heavy objects. I assume that "heaviest of all objects" (which if interpreted literally would be something like a supermassive black hole?) is meant metaphorically or as hyperbole.
But will falter to the lightest of all objects
The lightest of all objects is hydrogen. This could be referring to a chemical reaction that takes place when carbon comes in contact with hydrogen. Alternatively, graphite, also made of carbon, is one of the softest materials, so this could refer to a pencil coming in contact with a sheet of paper (which is among the lightest of objects on the scale experienced by humans).
I prefer long and skinny rather than short and thick
Pencils, which have a graphite core, are long and skinny.
since it is to my taste
which you cannot sense
Not sure about this.
You can see me as far the eye can see
in the seven seas
or the seven continents
Carbon is one of the most abundant elements, both on Earth and in the universe. Wherever you are and in any direction you look, you will see it.
Precious by nature
yet never rare
I seem to be more desired than gold
Diamonds, one of the forms in which carbon is found in Nature, are precious and more desired than gold. Yet carbon is abundant and not rare.
You are here because of me
and I am certainly not here because of you
All life on Earth is carbon based. Carbon existed long before humans, or even the Earth, did.
As a group, I can be a force to reckon with
Carbon belongs to a group of elements known as the carbon group, which also contains important elements like silicon and germanium, very useful semiconductors, and tin and lead.
yet I can be gentler than a breeze
as every organism adores me
Not sure what the first line is referring to. Organisms need carbon to survive.