I recently read the book "The Live You Can Save" by the philosopher Peter Singer. He said that yes, it is considered tacky and self serving to let the world know you did something charitable. That leaves us all in the position of not knowing what is normal, or perhaps that giving $1000/year is good enough. But if you learn that many of your coworkers give $20000/year, it will likely cause you to reevaluate if your $1000/year is really generous enough.

So, to turn it around on you, if your choice is to let friends know that you've donated $X to some charity which may cause them to also be more charitable, or to keep it private because you are afraid of looking tacky, you have made the choice that your self-image is more important than the extra money it may cause friends to donate to the needy.

BTW, it is a great book -- the most important book I've read in at least 10 years. One important point is his philosophy: all suffering is important, not just the people you know or who are local to you. Raising $30,000 for a fancy truck for a disabled person is unquestionably a kind act, but that same amount of money could save 20 lives in a developing country, or prevent river blindness in more than 100 children.