I believe that this question is not a closed-ended one.
And here's why:
Let's take who examples
a. you create web2.0 and then go order a fiverr gig to build 500 comments to that web2.0
b. you create web2.0 and then build 5 web2.0 with uniqe texts linking to this one and build 5 forum links also to this web2.0
While both a. and b. options can be considered as tier-2 link building, they are completely different.
In case a. your web 2.0 may even not get indexed because all those comments are from link dumps which also will never get crawled/indexed by Googlebot
In case b. your initial web2.0 is likely to get indexed and even gain some PA
My advice is to shift from "works/doesn't work" paradigm to setting objective goals.
Which goals?
For example to index a given page. Or to grow it's PA to 10 for example.
It's important because these are micro kpi's which will help you to build your SEO roadmaps and get rankings. Otherwise it would be betting, not SEO.
Sincerely hope, it would help you.
Good luck with your beginnings!