Depends on what the sites were? If they say stuff like "get 3 percent back daily!" or something like that its either a scam or a HYIP which is a type of ponzi scheme that will eventually run out of money. Now you can make a lot of money of these but you can also lose all your money easily.
There's investments like coin gambling sites were you invest to become part of the overall bank and receive part of all the profits from the site. These are nice and probably safer but you won't get rich from it unless you have alot to throw in.
There's also investments such as Bitcoin lending which seems pretty ok but I haven't tried much of it.
And finally there's the stock market as there are a lot of stocks you can get with btc these days.

Just be smart when it comes to investment and know what you are sending you money to.
You can check my blog in my sig to see what I've been using with success if you want.