757

757 is:

a palindrome  (a palindrome is a number that is the same backwards or forwards)

prime (a number with exactly 2 factors: 1 and itself)

happy (a number where, if you square all the digits and add the squares, then repeat with the sum, you will eventually get to 1.)
...start with 75:.  7^2 = 49,  5^2 = 25, 7^2 = 49.  49+25+49=123
...now we have 123:  1^2=1, 2^2=4. 3^2=9  1+4+9=14
...with 14:  1^2=1, 4^2=16.  1+16=17
...with 17:  1^2=1, 7^2=49.  1+49=50
...with 50:  5^2=25,  0^2=0.  25+0=25
...with 25:  2^2=4,  5^2=25  4+25=29
...with 29: 4^2=16, 9^2=81  16+81=97
...with 97: 9^2=81, 7^2=49.  81+49=130
...with 130:  1^2=1, 3^2=9, 0^2=0.  1+9+0=10
...with 10:  1^2=1,  0^2=0.  1+0=1  VIOLA!!!

These facts were posted by my Facebook friend Kevin L. Schwartz when his page had 757 "like"s.  He got the facts from Wikipedia.  I added in the explanations.