What Features, UX considerations make a Social Media Network Successful?

I agree with your ideas Mark, it seems to me that one should not aim too high, and be too generalist and probably avoid fighting on the same ground as the giants, or if one is reckless, then, do it differently and better.
For that it is better to be specialized in something, as you say, something around which the members can meet and exchange.
My idea was to mix the advantages of a social network and those of a dating site. I limited myself geographically to francophone countries.
So the idea is to tell members ok, you have Facebook to find people not far from you that you already know, come to us to find people you do not know yet.
Through your sharing and discovery, show who you are and see who the others are, find people who match you, talk to them, and meet them.
It works pretty well, I'm happy. 8 years after it went online, my site is now known and appreciated.
But it must be said that there is no miracle, it is really many hours of work, it is not enough to put a site online and wait with arms crossed.
The first day we were 5, me, and some friends that I had invited, today we are 20,000 and I never made any publicity for the site. It just works on word of mouth.
For that, as you say, you have to be serious, observe the shortcomings of the competition, and try not to reproduce them.
For example, the competition uses false profiles (50%). They are free labor, they drive up the meters, generate a lot of mail and are present at all hours. Ok, yes nice advantage, but in the short term only. Once people have understood that half of the members are fake they are fleeing, do not come to read their mails anymore.
I did the opposite, today, thanks to that I got the reputation of a serious site. Fewer members is true, less mail is true, but of better quality, it's also what you say, Mark.
I had to spend a lot of time on this, because it is also necessary to make the members understand that it is useless to receive 10 mails per day and to throw 10 to the trash. Better to receive two or three.
So I work a lot in rehabilitation, which means that I am always present and available.
I use forums to discuss all this, which makes a lot of progress in SEO, so it is done naturally.
Most dating sites are just a directory, three-button sites, with no real content, so even though they are mostly multinationals, it's "relatively easy" to take their place SEO point of view.
So do not hesitate to face the competition, especially if has a lot of shortcomings, just do things better than it.
Now if you dream of becoming "Zuckerberg II", you have a high probability of being disappointed, you will never get the CIA funding he got.
In this regard, many people are looking for "Free" and sites without ads. Here too, I made the difference. Most competing sites assume that you have to empty the visitors' wallet as quickly as possible because they know that with this principle, people will do the experiment once, maybe two, but not more.
I opted for another approach, my site is free and functional at the "Standard" level, which allows me to be in first position if we look for "free dating site". I remain honest, because my site can fulfill its mission for free.
I invite only the "standard" members to support us, without obligation, and in thanks, I give them more possibilities and more comfort.
Ok, I will never ride in Ferrari as well, but this system allows me to cover my expenses, and even a little more. And since all of this is just a passion for me, it's perfect.
The result is that I have loyal members, sometimes they leave, critics are essentially technical "Dolphin becomes old, only shadow on the board", but the important thing is that they do the trick and that in spite of that they always come back.
So now, I think my experience is a good illustration of what you said Mark :-)

I feel that if one is attempting to reinvent Facebook, that is the wrong route to take. Zuck caught some luck and Voia! Facebook. However, it's important to focus your energy on something you love or are truly interested in. Build that niche and keep the community interested with quality data and they will spend their time there. Nothing needs to be moving all around like back in the day of macromedia flash and you don't have to have so fancy of an interface...people want simplicity and responsiveness. If one can build a solution offering interesting data that is formatted and works well on any device, your community will undoubtedly grow exponentially.

good experience Baloo thanks for sharing