One thing which has been obvious to me for a long time is that the social networks which dominate today didn't succeed because they were technically better than the opposition. For my sins I am still on Facebook, and using that system is quite a battle. The interface is one of the worst I've ever encountered. The Twitter user interface is not quite as bad, but it still lacks features which other comparable systems have.
So why did Facebook and Twitter become the main social network systems?
First mover advantage
I think this is by far the biggest reason. Being first and being the thing which people get habituated to conveys an enormous advantage. Once habituated, even if the user interface is full of foibles, anything else with a different workflow will appear to be wierd, awkward, "not normal" and "hard to use".
Software is complicated and often there's a non-trivial amount of learning to get fully up to speed with how it works. That's a real cost in terms of time and effort, and not something that most people want to do often, or have the free time to do. Also the harder the learning curve the more likely you are to become highly committed to using a particular system, due to the sunk cost.
Mean time to profile twiddling
There's a rule of thumb that I have for judging whether social network software is going to get mainstream adoption and that's the amount of time between thinking "I want to join this system" and having an account and beginning to twiddle with your profile settings (uploading a photo, filling out the bio, etc). If that time interval is more than a couple of minutes then 99.9% of people are not going to bother.
If your system has a name which is hard to locate in a search engine, such as "Red", then this guarantees that the time to twiddling is going to be much longer on average.
If it's hard to find an instance to join or if you have to install an instance yourself then this also increases the time to twiddle by a big factor.
This means that the onboarding process is fairly critical and optimising that for searchability, minimum number of clicks, minimum cognitive workload and so on can have a big effect.
This is the thing which everyone knows about. No matter how jazzy the features you're not likely to join a system which your friends aren't on. The only time you are likely to do that is if you don't have other options. If you get purged in one of the many Facebook mass expulsion events, for example, then you're in a situation where you have to try out new things and find a new crowd to hang around with. But most of the time humans are herd animals and will stick with their familiar group.