My guess is that the number has recently increased, with most phone plans now offering unlimited talk+text but metered data. On a prepaid plan, when you've run out of data, some carriers will just do a hard stop on your data and others will reduce it to a slow trickle, so the SMS interface might be the best.
I very much doubt it. Using Twitter is such a way would be absolutely awful these days, and few people (outside of the tech bubble) hit their data plan limits.
It's not about how much data it hogs -- it's that for prepaid plans (which are the vast majority of overall subscribers worldwide), when your data runs out it often just completely turns off until you get another top-up card.
