This is SO important to me, as someone who is very active in my fandom and loves writing. For years when I first got into fanfiction, I never used to comment. I never commented, even when I really wanted to. Even when I had a lot to say. I was nervous and scared a lot, and it wasn't until this past year that I gained the courage to start commenting freely, and start showing excitement for the fics I liked. And as a result, I've really fallen in love with commenting! I've had authors be overjoyed to read my comments, I've had authors hint at plot-twists and future plot points, I've had authors respond with comments 3x longer than my own, I've had authors not respond at all the first couple times I leave a comment on their fics but later tell me that they're happy to see me under all the fics they post! Recently, I had commented saying "I would LOVE more fic!" on a fic that hadn't updated in 2 years, and you know what happened two weeks ago? They updated!
And when I get comments under my fics or Tumblr posts? I'm ecstatic!! I don't always respond, but that's not because I don't read or appreciate the comments, it's due to my own mental health, and responding to comments is something that I try to do better at, actually, because every comment is someone putting themself out there! They mean so much to me!! Not too long ago, I wrote a whole series of oneshots based off of a song album i really liked, and was so anxious about it not getting any kudos or comments. And you know what? I was right. I didn't get many kudos, but I did get a few comments, and they made my whole month. I still haven't forgotten about those comments, because of how important to me they are. If I hadn't gotten those comments, I probably would've taken a break from writing, but I did get those comments, and they made the 10 straight hours I spent working worth it despite my initial disappointment. On my tumblr posts, I go through my inbox every day, read every notification, but the one's that always stand out to me are the reblogs. The comments. The asks. I love them so much, the amount of interaction I get genuinely keeps me going when I would have otherwise quit months ago.
Not to mention: I notice all my consistent readers. I notice when they're some of the first to kudo a new fic, and I even recognize them under other people's fics! I have usernames and profile pictures memorized. I notice when a follower on Tumblr consistently likes my posts, or comes back every so often to read through my recent posts and like them! I notice that! And it makes me so happy every time!
Obviously, not every author is like me. Some don't need kudos or comments to keep going. Some will never finish a fic no matter how many comments there are. I've seen fics with hundreds of thousands of words and hundreds of chapters, and like 10-20 kudos and 2 comments. I've seen unfinished works with hundreds and thousands of kudos and hundreds of comments. But that doesn't mean your comments and likes mean nothing. They may mean more than you'll ever know! Worst case scenario, you leave a comment and nothing happens. Best case scenario, you convince someone to finish their first multi-chapter fic, or you inspire someone to keep writing and posting, or an author comes back after a years long hiatus.
Leave that comment saying 'I'm so excited for the next chapter/work!'
Guess the plot-twist the author is planning.
Quote sections of the chapter you liked the most.
Send a comment that's nothing but keysmashing and screaming.
Send a comment of just '!!!!'
It means so much more than you could imagine!