You still qualify for the exemption if you provided more for her support than she did. The amount she got from social security (or any other source) is not support unless she actually spent the money for support.
For example, if your daughter received $2,400 in social security benefits, and she paid $1,000 for lodging and $400 for recreation, that would mean she spent only $1,400 for her own support, even though she had income of $2,400. If you spent more than $1,400 for her support and no other support was received, you have provided more than half of her support.
For the schedule the IRS uses to determine support, and for the rules as to what is considered support, see IRS Publication 17, Your Federal Income Tax, which you can download at the IRS site at:
http://www.irs.ustreas.gov/forms_pubs/index.html