Well StatusNet exists, and has not devolved into anything.
What you want is to somehow have the tremendous amount of money to force people onto your platform (ie, what Google did with G+, but failed) without the profit motive to eventually turn the service into advertising media. Which basically means you want someone or some people with a lot of money to just throw it at userbase.
StatusNet is not "popular" for the same reason a lot of the FSF movements efforts are not popular - there is no advertising. Firefox and VLC are probably the most popular end user facing free software projects and they only exist in their current form because of word of mouth. They did have some substantial advertising efforts throughout the years, more than desktop Linux or other free software like Mumble ever has, and it takes that and just organic growth of userbase to become popular.
And even then, they were popular due to features, not freedom, though a lot of the features came from the freedom, and they were superior to their competition often because of contributions from the community.
Point is, you won't have a for profit social network get popular any other way, because your monetization options are limited when you are making it for profit, and you can only pump the absurd amount of advertising money into it to get a userbase by being for profit. The other alternative is StatusNet.
Firefox and VLC are popular because they had a superior UX compared to the proprietary competition. Firefox had plugins,tabs,and had more web features than IE. VLC,well, you don't need to care about installing codec A or B with VLC to read video files, it just works.
A opensource solution or free software, in order to win,must not only be free, but beat the competition in terms of user experience,ease of use,design...
I don't know, but I'd like to be on the team that tries to build it. I'd like to also try something decentralized but perhaps with small user run servers popping in and out of existence hosting niche content, and larger corporate servers in the mix distributing monetized content. I dare to dream.
I think it is possible to make some profit doing so. What would wrong is demanding that profits always increase. As a side note, Twitter really isn't all that bad IMO.
