Quick back of the envelope calculation... Lets assume 10 texts sent per person per day worldwide on average. Lets also assume everyone has a phone (far from true, but hey). Assume an SMS message is 160 bytes, but with compression is significantly less, but there is also connection/presence metadata - so cancelling out. So we have 1600 bytes * 7 billion people. Thats spread across 66 satellites. That totals 2 megabytes per satellite per second. Very doable.