I saw the Under the Radar event mentioned here at TechCrunch regarding Fox Interactive announcing 'acquiring' 5 of the 32 companies that were at the event. Pretty impressive for these web 2.0 start ups.
They actually mentioned they weren't acquiring Tagged, I had a quick look at this and it looks very similar to something a lot of my university friends use called Bebo and hi5. All very high use youth targetted social community sites sharing pictures etc. Myspace is very similar but has a more blog focus.
There was actually a lot of similarities between these web 2.0 sites:

• Calendar sharing and syncing with mobile devices (AirSet, CalendarHub, goowy, Mosoki, Rallypoint, skobee, zvents)
• news feed sharing (nowpublic, tagcloud, tailrank, top 10 sources, wink, dabbledb)
  
• blog sites (Box.Net, DogSter, Popist, zoho)
• search engine based community enhancements (Browster,  Eurekster, Kaboodle, rollyo, simpy)
• job search (simply hired)
  
• photo sharing (BubbleShare, Flock, riya, spymedia)
• podcast (Loomia)
• others (Meetro, sphere, formsassembly)

I guess every one has a chance of dominating a particular space...but can they compete against Yahoo, MSN and Google. Who can simply roll out a similar idea with their current user base and take over?

• Instant messenging is still all over the place with Skype, MSN, Yahoo, GTalk, AOL and plenty of others.
  
• Blogging is still all over the place with Blogger, MySpace, ...
• Photo sharing with Flickr, Yahoo, Google, MySpace, ...
• Online spaces from MyYahoo, Google, MSN, ...

I guess the calendar one hasn't been totally taken off yet...but then not everyone's friends are online and check every day. Blogs and Photos are different because they don't need a response, they are just there for reference. The "my" type sites only require the person who is online to be there. I'm really not sure Calendars will take off completely.