That isn't a fair comparison because people outside of Cruz's district are not able to vote for him. The same is not true for Twitter users and Twitter.
Is that really a fair characterization though? It may also be that many people feel obligated to use Twitter because it is already one of the "winners" and may not go anywhere due to network effects. So not engaging on it is the same as being silenced/unheard. Similarly, I don't think it would make sense to say "a billion users choose to use Google" and [N] voters voted for [politician].
I don't think it's any less fair than implying that CEOs of companies should have to be elected
You could say the same thing about people living in Texas who didn't vote for Cruz. :-)
The point is that it's not a fair comparison. It's a retort revealing by absurdity the invalidity of the question.