It's another Q&A site, but at the risk of sounding biased, it doesn't stand up to FunAdvice.
... Why not?
1. No community - we try to make sure that members here have a sense of community ... friendship. We do this with the Community section, profile pages that you can converse on and FunMail ... Ask.fm doesn't have these options.
2. No moderation - if you're fine with being bullied and harassed online, then Ask.fm isn't such a bad place, but the administration certainly doesn't step in and assist with these kinds of issues ... trolling is an ever-present danger on Ask.fm.
2. No Gigs - of course, the option is still in it's infant stage on FunAdvice, but once the word gets around, they will be a great way to make money!
It's a lot like formspring if you remember what that was. You can ask anonymous questions to your friends.