As explained earlier, you are able to give your friends assistance while playing Avengers Alliance, and a good tactic for these it to exchange S.H.I.E.L.D. points with each other. These you can give for free and so are a great way for advancing through the game. Once you reach the later levels though, S.H.I.E.L.D. points become less useful, so you may choose to exchange Energy points with your friends, as these come in very handy later on.
Gifts Not Working:
If you are unable to accept your Friends' gifts on Facebook, you have probably reached your Gift limit for the day. This is to stop people with hundreds of friends that play the game from being given a huge advantage. You can only accept 50 gifts per day. If this is the case you should see a message saying 'Gift Limit Reached' at the top of the screen.