If you use tier system to build back-link then 1tier sites/web 2.0 properties are the most important. Which web 2.0 sites you would use for building your 1tier network ? Here's my pick,
01) Blogger.com
02) Wordpress.com
03) Weebly.com
04) Webs.com
05) Yola.com
06) Tumblr.com
07) blog.com
08) Jimdo.com
09) blog.co.uk
10) webgarden.com
I always used those first 8 sites and then also use few nofollow Web 2.0 properties like,
livejournal.com
Squidoo.com
I try to update these at least twice a month and I keep backup set updating once a month. What are the web 2.0 properties you use for this ?