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If the content's really good, social can have an excellent pay off.  If you're promoting mediocre "me too" garbage, it's just not going to work.  You have to be really honest with yourself about the client's site and content -- if it's not good enough to work in the social space, don't burn the hours.  Instead, fix the content problem or focus on building links like I noted above, since they largely bypass the editorial process.

Most freelance sites should have people willing to write for those rates if you're willing to buy in bulk or commit long-term to a writer.  If a writer can sit down and bang out a bunch of these, they can do it more profitably than if you're giving them 1 or 2 at a time.  I prefer oDesk but any reputable site should do.

There are plenty of sites ranking for competitive terms that use blog comment links as a huge component of their link profile, so yes, they still work.  They are effective for establishing link diversity (your approach) and also great for camouflaging a link profile (even nofollow links) -- making it harder for a competitor to sift through your link profile and spot the links that are really powerful and driving authority to your site.

Beyond those considerations, the real value I see in blog commenting is the potential to establish relationships.  If you comment thoughtfully and regularly on the blog of an influential player in your industry, you increase the likelihood of being able to acquire a legitimate link from them in the future, receive social mentions, or potentially create a partnership of some sort (sponsorship or affiliate arrangement).

If you do it right, there's some value there.  Just make sure you focus on your other (more effective) methods and use blog commenting judiciously.

Here are seven actual link-building tactics that don't require you to have amazing content:

1. Private blog networks - easily the most powerful "hands free" method of building links right now.  Outsource the writing of 150 word posts for $0.50 to $1.00 each.  Perfect for building deep links and anchor-specific links to sites that already have some domain authority.
2. Guest posting - gives you full control over the content in your posts and the sites you get links from. Allows you to target much higher quality sites than private blog networks but you won't have as much control over anchor text.  Also, not ideal if the site you're linking to is sub par, as your author links are likely to be reviewed.  If your site is legitimate, this is one of the most powerful link building tactics around.
3. Directory links - they still work and can be completely automated.  Use these to build up a base of links to a site.  I automate through a service that lets me schedule these out over an entire year (100 per month).
4. Blog commenting - you can easily outsource it, automate it via Scrapebox if you want, or do it manually.  It depends on your risk aversion and own business morals.  There are simple search patterns you can use to identify dofollow blogs as well.
5. Footer links from WordPress themes / plugins - less obvious but highly effective. I have a site that picked up over 3 million links last year from theme footers ("powered by" links).  If you're smart, you can automate this to vary anchor text on the fly.
6. Forum profile links - easily automated but clearly spam.  If you're concerned about potentially burning a site, I wouldn't use these extensively pointing to your own site.
7. Buy them - not through a network, but creatively.  You can sponsor open source software (can get very high PR links this way) or a meetup group, give to a charity, donate a prize for a community event.  If you do enough community related stuff, you may be able to spin that as a newsworthy story to the local press and nab a link there.

If you combine these 7 tactics, you shouldn't have much trouble ranking.  If your content is awesome, you'll start to accrue natural links once you get the ball rolling.

Hope you find this to be helpful.