Web 2.0 sites can be setup easily as a network (or part of it). No problem there but they are all PR0 and N/A links (And pretty much near zero on any other metrics of authority) so they have to be "juiced up" by other links. There are three approaches to this

A) blast either the immediate network of sites (tier 1 sites) linking to your money site or another set of "tier 2" networks linking to your tier 1 site )and so on up to 3 or 4 tiers) with bunch of crappy links to squeeze a little juice to build uo the tier 1

B) get high PR links to juice up the tier 1 links buy buying them or buying domains with PR

C) do a combination of A and B

Ya you can, setting up web 2.0s then posting content that has a link back to your site is perfectly fine. Just make sure to use very diverse anchor text in your links.

If you're thinking years of possible good business consider starting the blog networks from scratch(potential of fake PR purchase eliminated) or study the heck out of the godaddy auction domains that have linking history and current PR. First option is safer and buying PR domains for a blog network will need luck for relevancy and more than $10 is the domain is actually a good one. My experience, so I'll only go with new domains if I won't be re-selling PR for the sake of a fee. Cool stuff that works.

I see a lot of posts from people claiming the merits of 2.0 sites that basically say hey, why spend money on buying domains for a network when you can just set up a private network with free 2.0 sites? It's free, easy and ayone can do that. But there's not any power in it. Because each site you set up is PR0. Ignore the talk of "devhub.com is PR5, set up a high PR site there for free!" Yeah, devhub.com is PR5 (their home page), your free devhub site will be PR0.

With that said, there's nothing wrong with incorporating 2.0 sites in your overall SEO. I've set up numerous such sites. If you feel that site (or more specifically page that holds the link) topic relevance matters, then you can start a few in one particular niche and use them for some links for clients in that niche. Sure they'll be PR0, but you can post relevant on-topic articles with contextual backlinks, and while that won't be a huge benefit (because of PR0), it can't hurt. And you can use them to include some non-keyword links ("click here", "www.domainname.com", etc.) if you're of the mindset that Google wants to see link diversity, including non-keyword links.

A year back I did SEO for a few gate/fence businesses, and as a SMALL part of SEO I set up eight 2.0 sites related to just that topic. Free, only took a bit of time. Threw a few PR3/PR4 links at them. Now, a year later, one is PR3, two are PR2, one is PR1, the other four are PR0 (referring to the home page when I say the "site" PR). So they actually have a little worth beyond the burn and churn way people usually use 2.0 sites (automate the set up of 20 sites, post an article to each, and forget about them).

So, in short, I think web 2.0 has some merit. However, I think it's only a modest part of SEO, and I rely MUCH MORE on "real" sites that I paid money for, that I own, that have PR value. In other words, build a network from PR domains you buy, and use web 2.0 as a minor addition to your overall SEO plan.

I knew RankWYZ can help you with your need.
It's like rent a system to automated the blog network using Web 2.0 networks.
Like owning your own private blog network, but contain web 2.0 free networks.

I posted a method for registering expired Tumblr domains for free here.

You can use the method to acquire expired PR1-4 (and higher) Tumblr domains with existing PR and backlinks. This is a cheap way to start a web 2.0 blog network. Make sure you diversify your backlink profile, because a profile with 100% Tumblr domains would look unnatural.