1. What does it mean?
맞팔 shortens 맞팔로우. It describes reciprocal following on social media, or asks the other person to return a follow you have already made.
Also written or said as matpal
2. The nuance
A mutual follow does not guarantee friendship, replies, or likes on every post. 선팔 is following first; 언팔 is stopping a follow.
3. Examples
“사진 계정 새로 만들었는데 맞팔할래?”
I made a new photography account. Want to follow each other?
“맞팔한 분들이 올리는 산책 사진 보는 게 좋아.”
I enjoy the walking photos posted by people I mutually follow.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.