I think a) is the most unique - there are already quite a few companies going after b) (FengOffice, Zoho Project, etc), and c) seems like it would be more difficult to monetize. It also might be easier to develop and market a Facebook App, depending on your skill-set.
That's the direction I'm going in now, it just seems like I'd be able to get things done a lot faster with someone helping out on the front-end. Is it common for startups to have back-end devs also work on the front-end when they first start? Is it worth offering 20 or 30 percent, or is it better to just hack something together until you can afford to hire a designer?
How many users do you have? I use Sendgrid because their API is really easy and reliable, and I know they have scheduling capabilities. You could probably come up with a script to schedule the emails as soon as a user registers. MailChimp probably offers something similar with their API, but I haven't spent much time using it.
I think people aren't looking at the bigger picture here... In 5 years the internet is going to look incredibly different than it does today - regardless of Google's, Verizon's or the FCC's position on "Net Neutrality".
You should choose the product that best fits your needs. Also, do you really think Microsoft is less 'evil' than Google? I'm driven crazy every day by Microsoft products and their business practices!