Yeah, we didn't move in until the 2nd October last year and it got so frustrating. I don't know if its any earlier this year but if not you may as well use the time to get talking to people
I didn't join the Facebook group for last year because i'd deactivated my account, but after seeing how well it worked out for the other girl i'm really annoyed that i didn't! Don't make the same mistake i did!
Just talk to everyone, even if its just asking what course they are doing or what block/floor/halls they're in. You won't stay in touch with everyone you meet in Freshers but it gets the awkward first conversation out of the way and they remember who you are, so you can develop plenty of friendships
If you are feeling brave maybe ask if they are doing something, everyone is bricking it about being alone and it is really unlikely they will say no! You meet SO many people through others as well. Just meeting one person could mean you get to know their entire friendship group.
Your JCR will probably have a good freshers week planned, and the nights out are a great way to meet people. In my hall i knew that everyone would be going out so it was really easy to get down to someone elses kitchen for predrinks and meet people that way. I still speak to quite a few of them even though i've moved halls.
Don't just confine yourself to the people you are living with either. Obviously spend time with them, but i know people who get on really well with their flat and know barely anyone else and they regret not making an effort in the first few weeks. I live on a corridor so i'm guessing this will be harder to do in somewhere like Sherfield or Stenton which is in flats.