It helps me to organize my collection and sort it by genres, release dates, etc.

In my experience, when using the phrase "sort by X", "X" tends to be singular, albeit I'm regrettably not certain on the reason for this. I think I do have some vague ideas though.

Additionally, one can publish reviews in order to share one's opinion with fellow users or just for to express oneself.

"For" and "to" are incompatible here. If you want to use "for": "one can publish reviews...or just for expressing oneself."

However, it is pretty easy to exaggerateget carried away with it.

get carried away: lose self-control.
"Exaggerate" isn't used in this way. "Get carried away" appropriately conveys what you want to say here. Another alternative, just for your information: "to go overboard with it".

I have accidentally caome across thea forum where people were discussing the amountnumber of movies in their watchlist, and plenty of them asserted that they had hundreds or even thousands!

(1) "Have came across" is ungrammatical. The past participle of "come" is still "come", so the phrase is "have come across", in the same way it's "have eaten" and not "have ate".
(2) Alternatively, just the simple past is okay: "I accidentally came across a forum".
(3) Another phrase that also conveys "accidentally come across", just for your information: "I have stumbled upon a forum".
(4) "A" is more appropriate than "the", unless there exists only one specific forum in which people talk about the length of their watchlists.
(5) "Amount" is typically used for uncountable objects, while "number" is used for countable objects like movies.

I found it frightening and decided to stop expanding my own list, which is comprised of a bit more than thirty films and a few dozens TV shows.

When "few" precedes an amount, the following word will be singular. Some other examples are: "a few hundred" (not "a few hundreds"), "a few million" (not "a few millions"), etc. This does not apply for other words: for instance, we'd still say "a few chickens" (not "a few chicken").

It resembles a social media platform because people can follow each other.

"Media" is the plural form of "medium", so "a social media" is ungrammatical. You could say "a social medium", but "social media" is such a common set phrase now that it's more natural to say "a social media platform". Alternatively: "It resembles social media because..."

Yesterday, I was thinking about the next one and was clueless as to what I should read.

(1) It's not immediately clear what exactly it is that you're "clueless" about. It will be good to clarify.
(2) Alternatively, if you don't want anything after "clueless": "Yesterday, I was thinking about what I should read next and was clueless." This would also sound more natural than your original sentence.

Now, I know in which order I ought to read them in order to get the bemost of itut of them.

(1) I think the phrase you meant was "get the most out of (something)", though I could be misinterpreting.
(2) Be aware of what "it" is supposed to refer to. In this sentence, it is most likely referring to "books", which is plural, so "them" would be most expected.

Additionally, one can publish reviews in order to share one's opinion with fellow users or just for to express oneself.

You can also use "or just for expressing myself."

I have accidentally came across thea forum where people were discussing the amount of movies in their watchlist, and plenty of them asserted that they had hundreds or even thousands!

It resembles a social media because people can follow each other.

Yesterday, I was thinking about the next one and I was clueless about what to read.