Here's what I noticed on a SMP like Twitter.

People like to like, but they also are expecting some privacy, so they created bookmarks, yet people didn't use it as much even though it delivers on what they want.

From my perspective, they just like the concept of 'liking' and perhaps want to show the creator that they liked it, and perhaps be a part of the social number of likes in a post.

While not the reason why they made likes private on Twitter, it ended up being the solution in a sense as an alternative to bookmarking, where only those who made the post knows that you like their specific post, otherwise it's private.

Another idea I had I had in mind to approach this issues is to have three different types likes:

Heart (public).

Heart with dashed border (pseudo private).

Heart with solid border (fully private).

You can choose your default, and holding it in a post would allow you to choose the type of like you'd like to initiate.