Recently the IFTTT folks re-enabled Twitter support, apparently having worked through the API cap issue. I hope that Electric Imp can come to a similar agreement / workaround with Twitter, re-enabling easier tweet posting support without using third-party services.
Alternately, if there were a way for a user to input their consumer API key into a twitter node in the planner, this would go a long way toward making Imp -> Twitter easier for your users.
Is it possible to post facebook updates on my own profile using agents? If yes, can someone help me with the code? I couldn’t find any documentation for that.
tracer:
Say more about your error. The following may not help, but…
My experience after running a tweeting cat door for about a year is that the Imp is almost never the weak link; I think I had to reboot it only once or twice. 
Open.sen.se does goes down from time to time. Before you spend debug effort check their demo feed at http://twitter.com/SenseDemo and make sure the twitter stream is updating every minute or so.
Hope that helps some.
yeshabavishi:
I have not tried this, but IFTTT.com and their FaceBook agent may provide some ideas.
Good luck!