Considering that I personally enjoyed Twitter more when it was “just us nerds,” perhaps these services don’t absolutely need to “catch on” or “take over” to be quite valuable to us in much the same way HN is, despite being much less popular than e.g. Reddit.
Because statusnet is a software, not a service, it doesn't really play by the same rules. You can have one site running it that appeals to more niche demographics (like techies and nerds) and one that has more broad appeal.
http://status.net/
http://identi.ca