First you will need to convert the timestamp to an actual Ruby Date/Time. If you receive it just as a string or int from facebook, you will need to do something like this:
my_date = Time.at(timestamp_from_facebook.to_i)
Then to format it nicely in the view, you can just use to_s (for the default formatting):
<%= my_date.to_s %>
Note that if you don't put to_s, it will still be called by default if you use it in a view or in a string e.g. the following will also call to_s on the date:
<%= "Here is a date: #{my_date}" %>
or if you want the date formatted in a specific way (eg using "d/m/Y") - you can use strftime as outlined in the other answer.