I think that it is the only major leading source from where we can actually earn bitcoin
but i am not sure about trading
No its Not True that Signature Campaigns are the Only Way to Earn Bitcoins Because there are Many Other Ways you can Use to Earn Bitcoins Online that are as Follows:-
1) You can Try Out Trading If you have Knowledge about Trading and If you want to Take Risks...
2) You can Offer Some Type Of Services in Exchange For Bitcoins in this Forum....
3) You can be a Affiliate of a Bitcoin Program that Suits you and earn real Commissions for Every Sale Made By your Affiliate Link....