I suspect that more of the time people used to spend on forums, interacting with people they knew little or not at all, they now spend online with people they are more or less closely acquainted in their real lives. The old way to be visibly social online was forums. The new way: facebook/tweeter/instagram.
Like it or not, the most scarce resource around is people's time and coffee can buy only so much of it extra. As such, increased time input in one area online, often means less usage in another area.
I doubt that the quality of content and repetition can be the real cause. There's only so much that's absolutely new. Even professional bloggers who have a lifetime of wisdom to share from their personal and professional life will, one way or another, tend to push the same views on the world, just do it from hundreds of different angles, many of which will be nearly indistinguishable.