500mb is pretty standard for most mobile phone contracts, you should be fine to use facebook and general internet browsing, just be sure not to stream too many vdeos or donload any large files.
For normal usage it's unlkely you'll use up the 500mb allowance if you keep to less than an hour a day of browsing, and avoid picture heavy sites too often.
Also your mobile network may let you look at your remaining mb allowance if you try and log in on their website, and that way you could keep an eye on your limit (I find it handy).