There are legit free (without capital investment) bitcoin giveaways even on twitter but it's not like the "send me btc and I'll double it" scheme by scammers. The giveaways I'm referring to are competitions like meme, creating songs or other forms.

Adding to @cryptomaniac_xxx said, ask the bitcointalk community when in doubt. But it's better if you can already tell which ones are legit and not.