I'm a blackberry curve 8520 user from the UK and i'm on orange but my blackberry recently keeps switching from EDGE to GSM which means that I can't use any of the social networking apps or blackberry messenger. Is there any way to fix this?! Please help!

You may be in a bad cell area. Does this happen everywhere?

It used to be fine until a couple of days ago and it happens pretty much everywhere, i only get proper connection for about 2 minutes every day

Did you drop the BB? or get it wet?

nope its in great condition

Unless you loaded a new App a few days ago It may be helpful to reload the OS to see if this will clear up any problems. Look at the FAQ section on the forum site. Look at the JL_cmdr application for information on doing the reload. Make sure you have a full backup and copy any photos or other media to a file . I would use desktop manager to do the backup. JL will allow a clean OS to be loaded.
As I said unless your in a bad area or the BB is defective, this is a simple fix other than replacing the phone.

Another answer may be that BB's are designed to lock-on to the strongest signal in the area you're in; it's unlikely that you'll get 3G coverage everywhere in the country (or even in certain buildings), hence the unit will search for the next strongest signal/transport medium (e.g. UMTS, GSM, gsm, GPRS, gprs, EDGE, edge etc. - the lower case versions meaning that all but calling and SMS functions will be limited or unavailable until you move to an area with better reception/signal strength).

Hope that helps.

SMS (Text Messaging) is available on all signal types and it uses the voice network to send information. MMS & Data is what is not available in "lower-case signal types")

The differences are:

4G/H+/3G: Data & Voice services available, and can be used simultaneously
EDGE/GPRS: Data & Voice services available, however cannot be used simultaneously
3g/gprs/edge/GSM: Voice services only available (includes the sending and receiving of SMS). Data Services not available.
SOS: Emergency Mode Only. Only calls to the Emergency Number can be made. Other Voice & Data Services are not available.
X: No Wireless Signal. No Voice (including Emergency) or Data Services Available
OFF: Wireless Radio turned off. Emergency Calls can still be made via the locked screen. All other Voice & Data Services are unavailable.

For more information:
KB02334-Network status indicators from the GSM/GPRS/EDGE/3G/4G networks

--Phil