If I opened the door for you, what would you say, "thank you".
If you were obviously in a rush and only had a couple of items at the supermarket and I let you jump the line (queue) what would you say. "Thank you".
Of course, it's probably an instinct, you don't even think about it.
My parents drumed my "pleases" and "thank yous" into me from an early age. I have a friend, who actually gets into a strop, if she does not get a thank you.
She blanked me for weeks and I couldn't understand why, so I text her with two words "thank you" and we were friends again. What happened? We had lunch and it was her turn to pay and things got caught up with us leaving and running for trains and I forgot to say "thank you". She was right.
So if we do this in real life, why don't we do it on social?
When somebody sends you a connection request (and the are not pitching) say "thank you".
If somebody comments on your blog, say "thank you".
It's just good manners!