Answer to Question 1
There are a number of problems that could arise, listed here are a few possible answers. First it is an online survey, thus only those alumni who check this website would potentially be able to respond. It is based on volunteer response and those alum who are not currently working or working in a less prestigious' job would be less inclined to respond.
Answer to Question 2
The resulting bias is non-response. Responding to a survey (or not) is voluntary, and those who respond are likely to have stronger opinions than those who do not respond. This type of non-response bias can lead to systematically overestimating or underestimating the truth about a population.