Hey there, {First_Name}
(This message is dependent on what you are promoting, try to keep it personal yet direct)
I saw your comment on {Post Title}, and figured you are interested in {Topic Name}.
I have recently launched {Product/Service details}. I believe this would be something of value to you as {Mention Why}.
OR
I am hosting a free live webinar series about {Topic Name}. I think this would be of your interest as I will be covering {Add Details}.
Let me know if you are interested in learning more!
Cheers,
{Your Name}