I won't rely on fb searches to raise the number of fans. I will choose a name that will make users remember my page, then upload there an image showing the url/name of your site which is your intent (make some branding).
The branding is what Facebook is really meant for. If you want the keywords that should be for search engines. If you want your Fans to remember you for long time or forever you need a Brand and not a Keyword. Develop yourself as a Brand and forget the keywords (for Social Media, it also work for Search Engines as well). Once you will become a brand the people will find you by typing your Brand Name rather by keywords and that will be repeat.
for username: having a '-' in between the username though will be better plus also gives you an advantage over the other in remembering, however to rely much on the name of the page should not make much difference in searches in comparison to the other modes of SEO. To chose the name of the page please try to chose a more elaborate name of the page without hyphons which is more understandable for human users, search engine robots will adjust themselves if not.
[edited by: engine at 10:17 am (utc) on Dec 19, 2011] [edit reason] self promo [/edit]
If the site is for your company then name the page on your company name so that the brand and popularity of your company will increase. If it is only a website for giving the online services then you can use the keyword as the page name.