I would never hide anything from my husband. Not my FB friends, not anything. The way to engender a trusting marriage is to never hide anything from one another.
People don't hide things in their marriage unless they HAVE something to hide. It took me two bad marriages and a lot of crappy boyfriends in my younger years to figure that out. Learn from my experience.
You may want to sit down and talk to your wife and ask her what she is hiding (don't ask her IF she's hiding something because she obviously is.... or she wouldn't be hiding it ... you may wish to just ask her point blank
what it is she's hiding). Don't do it in a confrontational way, though, but just be open and willing to hear whatever she has to say.
I could never imagine hiding my FB friends list from my husband.
Of course, we all need our own space and things, and DH and I have our private times away from one another. I think that's healthy in a relationship. But to actively HIDE a list of names from him? Hmmm. Couldn't see myself pullin' that kind of shenanigan on my hubby. I love him too much to do something like that.
The only thing I hide from Farmer Lew are his presents until it's time to open them.
And sometimes if I spend too much money, I don't tell him until later that night or the next day. But I do tell him, and if he ever asked "How much did you spend today?", I wouldn't lie or hide it from him.