At current prices you can send about 500 messages, likes, tips per dollar give or take depending on the length of the message. But users often tip each other here as well.
The smallest you can give is about 546 (?) I think around there, that is the current dust limit on Bitcoin Cash, that would be a bit like paying a minimum wage in tip;P
The cost is measured in the number of bytes, it varies around 200 up to 400 bytes, each byte is 1 satoshi, so a message costs about $0.0025 at current prices (or 0.25 cent).