Web 2.0s are effective because you are piggy-backing off their authority and trust. Yes, it is a sub-domain and your page authority will start out with a goose egg ZERO.
But there’s one important thing to remember about these properties:
they build Page Authority (PA) FAST.
With only a few strong links, any given web 2.0 can go from a PA 0 to PA 30 +.
In addition to the authority, you are also getting super a super relevant link.
Relevancy is king when it comes to link building. Don’t forget that.