In my experience, web2.0 was and still one of the best off page SEO links and they give a an increase in the SERPs but it totally depends on what do you think about when you talk about a web2.0.
Are you refering to the shitty services that gives you a link from a web2.0 that was used by 1000 person before? If yes, it won't help you at all.
But, if you're willing to use Web2.0's and to get gold results from them, you must create a HQ Web2.0 that looks like a standing website.
Let say you're focusing on Tumblr, you must buy/collect some expired tumblr's to build your Web2.0 empire in (but for sure you must use different platforms) . After that, you should add some great content to it, and start building links to it (tier2).
If you're doing it the right way, you will have a push in your rankings.