Hi Kiran,
It would greatly help if you can post here the json data that you send to the service so we can take a look at it.
As for target->id, you should provide the id of the target within the account. You can get this from the GET /services.json call. Please also take a look at some example posted earlier on this forum.
We use two level identification for targets (1st level is account id, 2nd level is target id) in order to support user accounts that can post as different 'identities'. For instance, if you add your Facebook account, that account will support posting to your 'personal' wall but also you might post to a Facebook Page associated with your account. We call this different targets within an account. A similar case is with Tumblr where a user can have multiple blogs associated with his account.
Hope this makes sense,