M1's terms limit referral links to friends and family and bar publicly advertising M1 in any fashion.
What the clause says
M1 states that referral links should be used for friends and family only, and that eligible referrers may not purchase internet search terms or publicly advertise M1's products or name in any fashion. A separate sentence asks that you send your link only to people you know personally.
The part that bites
M1 goes further than most by making the person you referred bear the cost too: new users who sign up through a link found on social media are stated to be ineligible for the bonus. So a publicly posted link can fail for the person who used it, not only for the person who posted it.
Cite the live page, not the PDF
M1 hosts two referral terms documents on its own domain and they disagree. A standing PDF still describes a ten dollar referral bonus, while the live promotion page describes seventy five dollars. Both are on m1.com, so the usual verify-at-source shortcut of trusting the official domain is not enough here. The live promotion page is the one to read.