Links are weighed more on their attributes than their categories. Rather than trying to get social bookmarks or web 2.0 links in particular you should be looking for tier 1 links that:
1. Are d0follow.
2. Are on a page with less than 20 OBLs (the fewer the better).
3. Are on a page with PageRank.
4. Are on a page that's domain has PageRank.
5. Are on a page that's indexed in Google.
6. Are on a page that's niche related.
7. Are on a page with unique content.
8. Can use targeted keywords as anchor texts.
If you just look at the broad category you're not getting the full picture.