משום דרכי שלום, tzedakah can be given to all people. Welfare and food stamps should be then defined as tzedakah as it's given to all people under the poverty line, Jewish or not. Medicaid should generally be considered tzedakah too (although I'm not sure about the LTC part). If it's tzedakah, is it considered maaser?
Rav Moshe Feinstein zt"l in Igros Moshe (cheilek 2 - first volume on Yoreh Deah), Siman 143 says that tax that goes to a government is not considered to be part of one's personal funds as they are owed outright to the government. So with this in mind, one could say that even if the tax goes towards a modern welfare system, since it is not deemed to be part of your personal income you wouldn't be fulfilling your obligation of maaser.
He writes clearly:
המסים של האינקאם טעקס שזה הוא מס מהרויח אין צריך לעשר עליהם משום דנחשב כלא הרויח זה כלל
"The tax of income tax which is a tax on one's profit, should not be tithed on them because it is considered as non-profit - this is the rule."
However, Rav Yitzchak Weiss in Minchas Yitzchak (cheilek 5) 34:9 who mentions the above ruling of Rav Moshe disagrees and says that this is a stringent approach and if a person receives a minimal / low income he can consider the tax payment as part of his maaser obligation.
אבל למי שאין פרנסתו ברחבה כ"כ יוכל לנהוג כן בכל מסים שלא לעשר מהם ויתנה כן מתחלה מ"מ אם העשתא דחיקא לי' יכול לנהוג כן בהמעשר שעדיין לא הפריש
"But someone who doesn't have such a big livelihood / income, he is able to treat it like all taxes that he doesn't tithe them, and he can stipulate so at the beginning. Nevertheless if it is pressing for him, he is able to treat it as maaser since it still has not been designated."