SEO Tactic: Social Media backlinks

To make sure your links come from a variety of different types of website you can mix up your link-building by
creating accounts and profiles on major Web 2.0 properties.
You can also add websites that are niche-specific. Just create an account and add your link in your profile where possible, and then use those sites as applicable to submit your URL.

Below is a list of some of the main ones:

HubPages.com
Tumblr.com, 
wetpaint.com, 
zimbio.com, 
weebly.com, 
blog.com, 
blogger.com

This is only a sampling as there are hundreds of other Web 2.0 sites you can go to. See below for a larger list

Find Niche-Specific Social media platforms: This link has many specialised niche social networking sites:
http://socialmediaanswers.com/niche-social-networking-sites/

Use the "Google News Alert" for this term: “new social network” and have the alerts sent to your inbox everyday.  Set up both news and blog alerts.  
Doing this will make sure you are kept informed of new sites launching. You will also be one of the first to get backlinks from them - a good thing

Sometimes you can find bargain advertising prices when they sell advertising.  Many of them will be happy to give you great deals.
This is another goldmine because you’ll be directly matching your offer to people who are interested/passionate/excited/etc. about the niche you’re trying to communicate with.

Top 50 Web 2.0 Websites:

1)  http://www.youtube.com/
2) http://www.wikipedia.org/
3)  http://www.craigslist.org/about/sites
4)  http://twitter.com/
5)  http://photobucket.com/
6)  http://www.flickr.com/
7)  http://wordpress.com/
8)  http://www.imdb.com/
9)  http://digg.com/
10)  http://www.ehow.com/
11)  http://www.typepad.com/
12)  http://www.topix.com/
13)  http://www.livejournal.com/
14)  http://www.deviantart.com/
15)  http://technorati.com/
Also from: http://movers20.esnips.com/TableStatAction.ns?reportId=100
16)  http://www.myspace.com/
17)  http://www.facebook.com/
18)  http://www.linkedin.com/
19)  http://www.hi5.com/
20)  http://www.joost.com
21)  http://www.orkut.com/Signup
22)  http://www.tagged.com/
23)  http://www.ning.com/
24)  http://www.friendster.com/
25)  http://www.metacafe.com/
26)  http://www.fotolog.com/
27)  http://www.wikia.com/wiki/Wikia
28)  http://multiply.com/
29)  http://www.stumbleupon.com/
30)  http://www.squidoo.com/
31)  http://www.skype.com/
32)  http://www.last.fm/
33)  http://www.bebo.com/
34)  http://www.etsy.com/
35)  http://www.reddit.com/
36)  http://www.vimeo.com/
37)  http://delicious.com/
38)  http://www.pandora.com/
39)  http://www.meebo.com/
40)  http://www.fotolia.com/
41)  https://www.google.com/accounts/ServiceLogin?service=feedburner
42)  http://www.woot.com/
43)  http://www.buzznet.com/
44)  http://www.meetup.com/
45)  http://www.zazzle.com/
46)  http://www.xanga.com/
47)  http://www.myyearbook.com/
48)  http://www.flixster.com/
49)  http://www.kaboodle.com/
50)  http://www.snap.com/