Navin told me that Google will not give Gmail invitation links to users anymore. He pointed me to a discussion forum of the KVPUG community, where I could read it all happens, thanks to recent Gmail exploitations and misusage. Some users don't get invitation links any more while others report missing invitation links. This sounds odd because I believe that Google will not take such actions, as quoted on the BBC news article:
Despite the policy change Google still seems happy for invitations for accounts to be swapped and traded.
So, I guess those are just rumours. As long as I'm on Gmail now, I don't care much anyway. What I care now is that I can finally import my contacts into Gmail! Yeah!