What is the very big deal if I want to build an application like Facebook?

Supposing that I Know how APIs works , and also able to design modern design

Facebook is a diseased mindset, not an application.

It's not a big deal, but, did anyone ask for another Facebook?

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@Demolishun yes me. For my community

Scope. Facebook is a massive site with enormous amount of content.

Which part where you interested in.

Trying to recreate it all will make the endeavor pointless as you community will be gone before your done.

What @Voxera said. There's a lot more to Facebook's engineering than a simple pile of scripts and html/css, and that's mostly because of the massive scale at which they operate. Look up some of the engineering talks their folks have given.

Besides, you have to offer something which FB or its alternatives (like the fediverse) don't do already.

Same for any other large scale service. So both the how and the why are huge challenges.

1) Try building a TODO list app. If you're good, it should take you perhaps an hour.

2) Try building a TODO list app that 2 people can edit at the same time. Now you have to handle things like race conditions, edit locks, polling updates and other things. Do you see how adding a simple extra condition makes the app 10 times more complex?

3) Now imagine making a TODO list app that a MILLION people use at the same time and think about how you'd handle all of those things. Doesn't seem so simple now?

These are the problems that an app on the scale of Facebook solves every day. If it seems simple to you, that is in and of itself an illusion created by how well they've designed it.

A feed with content - a day job

A feed with content from billions that doesn't crash in a heartbeat - many years with 1000's of developers.

But by all means, start small, you may hold the keys to killing Facebook once and for all.

You can't. You can deploy your app to AWS's infinitely scalable infrastructure, you can write it from the ground up to utilise serverless architecture as best as it can, but you can't possibly ever accumulate the knowledge Facebook has (not even talking about user data here). 15 years of some of the world's best techs, data analysts, product designers, testers, and millions of users providing their feedback about their work is simply impossible to replicate.

You can achieve the similar functionality, but do you know why FB made some button that colour? Why they placed the layout the way they did? Why they separated permissions in those exact access levels? Learning to build a rocket engine from scratch takes less knowledge & information than what went into FB.

Servers. Lots of them. And a software which can fetch any data from any of those servers very fast.

@RememberMe And I was not even talking about the technical problems but the scope if content type and services.

Even if the goal is a small scale community site where you do not even need multiple servers, facebook is so many different things.

The term social network is a lot more than just a feed of posts with comments and thumbs up.

So first priority is what part if the facebook site would you start with.

Then you come to all the technical hurdles :)

@Voxera agreed.