It sounds very natural to me. Both 'sometimes' and 'sometime' can be used, but the meaning changes slightly. They both ask you to message them, but the first is more a general comment/observation saying you need to message me from time to time.You need to keep in touch with me. The statement is implying that more than one future message is required. The 2nd is more a request asking for a message. It doesn't have to be from time to time (like the first) or several messages, but you do need to message me at least once .

"It wouldn't hurt to message me sometime" is what people would commonly say. Sometimes (with the "s") is used differently. The train is late sometimes. I get sick sometimes. There's an air of unpredictibility in it; it happens occasionally but might or might not. Sometime (with no "s") has more certainty; only the time is unclear. I will eat dinner sometime tonight. John will arrive home sometime after 7.

"It wouldn't hurt to message me sometime" is what people would commonly say. Sometimes (with the "s") is used differently. The train is late sometimes. I get sick sometimes. There's an air of unpredictibility in it; it happens occasionally but might or might not. Sometime (with no "s") has more certainty; only the time is unclear. I will eat dinner sometime tonight. John will arrive home sometime after 7.