Is there any way to hide some info?
Apparently not much can be done so far. You can still try to "hide in the crowd" by adding noise to your profile.
For instance, if you join Mi Yodeya (judaism.stackexchange.com) and you are worried your colleagues will be able to see you have a profile there, you can then join Islam and Buddhism and Hindouism and Christianity. Why not then join Ask Different and Ask Ubuntu and Unix and Linux, etc.?
Unfortunately, it simply delays the information. A few extra clicks will enable others to see how many questions you have actually posted there. Even after "hiding" all of my communities on my profile page, this is (part of) what any user can see on my profile:
As you can see, there's even the (approximate) date when you joined the community, and the "last seen" date. So any potential stalker can easily see in which communities you're really active.
To have an effective "shadowed" profile would require so much fake data creation (random logins and upvotes, dummy questions, etc.) that it does not seem currently reasonable.
On the bright side, at least this means StackExchange cannot actively monetize on this information, as in "for $X/month, we can show you all activity user Y produced on the entire network".