I think the reason you don't often see spam on GitHub is also a matter of convenience. While GitHub as a whole is very large, there are few people reading a spe

I think the reason you don't often see spam on GitHub is also a matter of convenience. While GitHub as a whole is very large, there are few people reading a specific issue, and posting spam elsewhere doesn't really make sense (nobody will look at your "buy-viagra-now" repo, nor will many people notice a new issue/PR called "Buy Viagra now"). And even then, you can assume that all readers are sufficiently tech-aware and won't fall for direct spam. From the spammers' point of view, you're wasting resources on a platform that has virtually zero chances of yielding visitors, as opposed to mail spamming which has higher chances in virtue of higher visibility and less conscious users.