Although Im not questioning fastmail ability to provide secured email service. Yet I cannot help think why I would rely on a small player for email. I would ask what is the probability of fastmail service getting hacked say compared to gmail.
(Somebody has right pointed out that ) Ad relevance to your email is not same as reading - thinking in same vein should also lead us to believe even (spam) classification can be termed as reading.
If Fastmail were, say, 1% of Google size, it would still be #2, which is a pretty good ranking on any market. And if you don't lose features by switching to the #2, please do it, for the sake of competition – Give freedom a chance.
I'm using Fastmail for my 3-ppl business. We lack no feature, we get used to it, we pay them (cheap), we don't leak our source code to GitHub and other browsing habits or contacts to Google Analytics/G+, it's just good management of the company information. We are the annoying guys who can't click "Connect with Facebook" nor "Connect with G+" and that makes providers keep the login/pw auth. Plus it funds a company so you might still have a choice in 5 years. And we show VCs that writing "privacy" on commercial documents yields customers.
Cost to us? Zero.
Fastmail and gmail both have mature bug bounty programs, which mean they will pay out cash to anyone who finds and reports a way to hack them: https://www.fastmail.com/about/bugbounty.html This shows that they are willing to put money behind their claims to security and probably puts them in the top few % of difficult to hack websites above eg windows live mail.
Also, it's a mistake to think a company will be more secure because it's larger. If anything, I'd say it's the opposite; large companies tend to have complex websites with vastly more attack surface.
Of course, every email provider is vulnerable to certain governments.
My opinion is tracking, ads are all fine on the primary website, but if you follow me around the web, tracking my every move, I'll stop using such services and try my best to prevent the tracking.
From their privacy policy:
"Incoming messages are scanned for the purpose of spam detection unless you disable spam protection for your account... To make message searching fast, we build an index of your messages (this is a table, just like you would find at the back of a reference book, in which you can look up a word to quickly find the emails in which it appears).
No information from any of these activities is used for any other purpose, or to compile any kind of profile on our users."
https://www.fastmail.com/about/privacy.html