Just like anything else in life, social media has a definite life cycle. That life cycle has several critical elements and one feeds off of the other.
The Social Media Lifecycle by Douglas Karr on MarTech.
No doubt people have studied seriously and written on this, but shooting from the hip here: 1. Beta / private / stealth / idea / inside the playpen pre-launch; 2. Every time a friend on your network bolts, a network becomes less valuable to you. That is another quality that will take time and nurturing. The changes are meant to increase user engagement on the site, make it easier to navigate on mobile phones and provide even more highly targeted advertising. Social network life cycle marketers we have to constantly innovate, so do the platforms if they want to keep up with consumer demand. Social media can help with the promotional strategies that assist with the product life cycle. It is more an emotion than an action. On your other point — I think the problem with many of the networks we have seen get huge and then start floundering is when they move out of their initial niche and try to be a network for everyone. SOCIAL MEDIA STRATEGY - MEMBERSHIP LIFE CYCLE