Although Flock has some nice features for integrating your social networks and media, it also has some very significant drawbacks. It always lags behind the current Firefox version for security updates & other fixes, and bookmarks work very differently (which I personally dislike).
Yoono is a Firefox plugin that gives you the best of both worlds. It adds a sidebar which provides much of Flock’s more useful functionality. Combined with Scribefire for blogging, you can have most of Flock’s functionality with a more mature & well supported Firefox browser.
Plurk is yet another site attempting to compete with Twitter. Although it has some interesting features, like the timeline view and the ability to share pictures & videos, it still seems lacking. Plurk doesn’t support SMS or IM, and the lack of an API means there’s no desktop client.
Twitter’s main problem is scalability and not being able to handle the user load. There’s no indication that Plurk or other systems can handle that load either. I’m more confident in Twitter’s ability to redesign, re-architect and scale their service, since they have the funding and bandwidth to support their growth.
I joined net4mac a few days ago, but so far I don’t see much point to it. This is a new “social network for Mac users”, but it’s severely limited by requiring their proprietary software. You can view public pages from a web browser, but to write anything or contact anyone, you need to use their software. This means you can’t access it from an iPhone (unless they release iPhone software).
If anyone’s interested, here’s my member profile.